doc/filters/crop: drop unmatched quote in example

Commit 55b81528a9 ("doc/filters: itemize crop examples") dropped the
quotation marks from these examples, but this one remained. Quotes are
actually needed to put the example into a command line or a program, but
removing it here makes the example consistent with the document.

doc/filters.texi

@@ -10697,7 +10697,7 @@ crop=in_w/2:in_h/2:(in_w-out_w)/2+((in_w-out_w)/2)*sin(n/10):(in_h-out_h)/2 +((i
 @item
 Apply erratic camera effect depending on timestamp:
 @example
-crop=in_w/2:in_h/2:(in_w-out_w)/2+((in_w-out_w)/2)*sin(t*10):(in_h-out_h)/2 +((in_h-out_h)/2)*sin(t*13)"
+crop=in_w/2:in_h/2:(in_w-out_w)/2+((in_w-out_w)/2)*sin(t*10):(in_h-out_h)/2 +((in_h-out_h)/2)*sin(t*13)
 @end example
 
 @item