doc/filters/histogram: copyedit for grammar

There were a few grammar and spelling mistakes in this filter
description. I went through it and corrected them without changing
the meaning.

doc/filters.texi

@@ -4937,8 +4937,8 @@ the histogram. Possible values are @code{none}, @code{weak} or
 
 Compute and draw a color distribution histogram for the input video.
 
-The computed histogram is a representation of distribution of color components
-in an image.
+The computed histogram is a representation of the color component
+distribution in an image.
 
 The filter accepts the following options:
 
@@ -4949,41 +4949,38 @@ Set histogram mode.
 It accepts the following values:
 @table @samp
 @item levels
-standard histogram that display color components distribution in an image.
-Displays color graph for each color component. Shows distribution
-of the Y, U, V, A or R, G, B components, depending on input format,
-in current frame. Bellow each graph is color component scale meter.
+Standard histogram that displays the color components distribution in an
+image. Displays color graph for each color component. Shows distribution of
+the Y, U, V, A or R, G, B components, depending on input format, in the
+current frame. Below each graph a color component scale meter is shown.
 
 @item color
-chroma values in vectorscope, if brighter more such chroma values are
-distributed in an image.
-Displays chroma values (U/V color placement) in two dimensional graph
-(which is called a vectorscope). It can be used to read of the hue and
-saturation of the current frame. At a same time it is a histogram.
-The whiter a pixel in the vectorscope, the more pixels of the input frame
-correspond to that pixel (that is the more pixels have this chroma value).
-The V component is displayed on the horizontal (X) axis, with the leftmost
-side being V = 0 and the rightmost side being V = 255.
-The U component is displayed on the vertical (Y) axis, with the top
-representing U = 0 and the bottom representing U = 255.
-
-The position of a white pixel in the graph corresponds to the chroma value
-of a pixel of the input clip. So the graph can be used to read of the
-hue (color flavor) and the saturation (the dominance of the hue in the color).
-As the hue of a color changes, it moves around the square. At the center of
-the square, the saturation is zero, which means that the corresponding pixel
-has no color. If you increase the amount of a specific color, while leaving
-the other colors unchanged, the saturation increases, and you move towards
+Displays chroma values (U/V color placement) in a two dimensional
+graph (which is called a vectorscope). The brighter a pixel in the
+vectorscope, the more pixels of the input frame correspond to that pixel
+(i.e., more pixels have this chroma value). The V component is displayed on
+the horizontal (X) axis, with the leftmost side being V = 0 and the rightmost
+side being V = 255. The U component is displayed on the vertical (Y) axis,
+with the top representing U = 0 and the bottom representing U = 255.
+
+The position of a white pixel in the graph corresponds to the chroma value of
+a pixel of the input clip. The graph can therefore be used to read the hue
+(color flavor) and the saturation (the dominance of the hue in the color). As
+the hue of a color changes, it moves around the square. At the center of the
+square the saturation is zero, which means that the corresponding pixel has no
+color. If the amount of a specific color is increased (while leaving the other
+colors unchanged) the saturation increases, and the indicator moves towards
 the edge of the square.
 
 @item color2
-chroma values in vectorscope, similar as @code{color} but actual chroma values
+Chroma values in vectorscope, similar as @code{color} but actual chroma values
 are displayed.
 
 @item waveform
-per row/column color component graph. In row mode graph in the left side represents
-color component value 0 and right side represents value = 255. In column mode top
-side represents color component value = 0 and bottom side represents value = 255.
+Per row/column color component graph. In row mode, the graph on the left side
+represents color component value 0 and the right side represents value = 255.
+In column mode, the top side represents color component value = 0 and bottom
+side represents value = 255.
 @end table
 Default value is @code{levels}.
 
@@ -4996,8 +4993,8 @@ Set height of color scale in @code{levels}. Default value is @code{12}.
 Allowed range is [0, 40].
 
 @item step
-Set step for @code{waveform} mode. Smaller values are useful to find out how much
-of same luminance values across input rows/columns are distributed.
+Set step for @code{waveform} mode. Smaller values are useful to find out how
+many values of the same luminance are distributed across input rows/columns.
 Default value is @code{10}. Allowed range is [1, 255].
 
 @item waveform_mode
@@ -5016,26 +5013,25 @@ It accepts the following values:
 @table @samp
 @item parade
 Display separate graph for the color components side by side in
-@code{row} waveform mode or one below other in @code{column} waveform mode
-for @code{waveform} histogram mode. For @code{levels} histogram mode
-per color component graphs are placed one bellow other.
-
-This display mode in @code{waveform} histogram mode makes it easy to spot
-color casts in the highlights and shadows of an image, by comparing the
-contours of the top and the bottom of each waveform.
-Since whites, grays, and blacks are characterized by
-exactly equal amounts of red, green, and blue, neutral areas of the
-picture should display three waveforms of roughly equal width/height.
-If not, the correction is easy to make by making adjustments to level the
-three waveforms.
+@code{row} waveform mode or one below the other in @code{column} waveform mode
+for @code{waveform} histogram mode. For @code{levels} histogram mode,
+per color component graphs are placed below each other.
+
+Using this display mode in @code{waveform} histogram mode makes it easy to
+spot color casts in the highlights and shadows of an image, by comparing the
+contours of the top and the bottom graphs of each waveform. Since whites,
+grays, and blacks are characterized by exactly equal amounts of red, green,
+and blue, neutral areas of the picture should display three waveforms of
+roughly equal width/height. If not, the correction is easy to perform by
+making level adjustments the three waveforms.
 
 @item overlay
-Presents information that's identical to that in the @code{parade}, except
+Presents information identical to that in the @code{parade}, except
 that the graphs representing color components are superimposed directly
 over one another.
 
-This display mode in @code{waveform} histogram mode can make it easier to spot
-the relative differences or similarities in overlapping areas of the color
+This display mode in @code{waveform} histogram mode makes it easier to spot
+relative differences or similarities in overlapping areas of the color
 components that are supposed to be identical, such as neutral whites, grays,
 or blacks.
 @end table