doc: Consistently use 'filtergraph'

We used to use 'filtergraph' or 'filter graph' randomly.

'filtergraph' was chosen due to having 57 occurrences in
the documentation, whereas 'filter graph' had only 19.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

doc/ffmpeg.texi

@@ -346,11 +346,11 @@ Use fixed quality scale (VBR). The meaning of @var{q} is
 codec-dependent.
 
 @anchor{filter_option}
-@item -filter[:@var{stream_specifier}] @var{filter_graph} (@emph{output,per-stream})
-Create the filter graph specified by @var{filter_graph} and use it to
+@item -filter[:@var{stream_specifier}] @var{filtergraph} (@emph{output,per-stream})
+Create the filtergraph specified by @var{filtergraph} and use it to
 filter the stream.
 
-@var{filter_graph} is a description of the filter graph to apply to
+@var{filtergraph} is a description of the filtergraph to apply to
 the stream, and must have a single input and a single output of the
 same type of the stream. In the filtergraph, the input is associated
 to the label @code{in}, and the output to the label @code{out}. See
@@ -496,8 +496,8 @@ stream
 @item -vlang @var{code}
 Set the ISO 639 language code (3 letters) of the current video stream.
 
-@item -vf @var{filter_graph} (@emph{output})
-Create the filter graph specified by @var{filter_graph} and use it to
+@item -vf @var{filtergraph} (@emph{output})
+Create the filtergraph specified by @var{filtergraph} and use it to
 filter the stream.
 
 This is an alias for @code{-filter:v}, see the @ref{filter_option,,-filter option}.
@@ -637,8 +637,8 @@ Set the audio codec. This is an alias for @code{-codec:a}.
 Set the audio sample format. Use @code{-sample_fmts} to get a list
 of supported sample formats.
 
-@item -af @var{filter_graph} (@emph{output})
-Create the filter graph specified by @var{filter_graph} and use it to
+@item -af @var{filtergraph} (@emph{output})
+Create the filtergraph specified by @var{filtergraph} and use it to
 filter the stream.
 
 This is an alias for @code{-filter:a}, see the @ref{filter_option,,-filter option}.

doc/ffplay.texi

@@ -73,19 +73,19 @@ Default value is "video", if video is not present or cannot be played
 You can interactively cycle through the available show modes by
 pressing the key @key{w}.
 
-@item -vf @var{filter_graph}
-Create the filter graph specified by @var{filter_graph} and use it to
+@item -vf @var{filtergraph}
+Create the filtergraph specified by @var{filtergraph} and use it to
 filter the video stream.
 
-@var{filter_graph} is a description of the filter graph to apply to
+@var{filtergraph} is a description of the filtergraph to apply to
 the stream, and must have a single video input and a single video
 output. In the filtergraph, the input is associated to the label
 @code{in}, and the output to the label @code{out}. See the
 ffmpeg-filters manual for more information about the filtergraph
 syntax.
 
-@item -af @var{filter_graph}
-@var{filter_graph} is a description of the filter graph to apply to
+@item -af @var{filtergraph}
+@var{filtergraph} is a description of the filtergraph to apply to
 the input audio.
 Use the option "-filters" to show all the available filters (including
 sources and sinks).