doc: replace '×' symbol with 'x'

Non-ascii chars are not correctly rendered in both MAN and HTML output or
may even cause compilation failure. Thus avoid to use them until we find
a satisfying way to deal with them.

Fix trac ticket #2243.

doc/ffmpeg.texi

@@ -1024,7 +1024,7 @@ ffmpeg -filter_complex 'color=c=red' -t 5 out.mkv
 
 As a special exception, you can use a bitmap subtitle stream as input: it
 will be converted into a video with the same size as the largest video in
-the file, or 720×576 if no video is present. Note that this is an
+the file, or 720x576 if no video is present. Note that this is an
 experimental and temporary solution. It will be removed once libavfilter has
 proper support for subtitles.
 

doc/filters.texi

@@ -5996,7 +5996,7 @@ Activate unsafe mode: do not fail if segments have a different format.
 The filter has @var{v}+@var{a} outputs: first @var{v} video outputs, then
 @var{a} audio outputs.
 
-There are @var{n}×(@var{v}+@var{a}) inputs: first the inputs for the first
+There are @var{n}x(@var{v}+@var{a}) inputs: first the inputs for the first
 segment, in the same order as the outputs, then the inputs for the second
 segment, etc.