doc/filters: fix typo

Found-by: bloody123
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/filters.texi

@@ -9384,7 +9384,7 @@ Set the number of output video streams, that is also the number of video
 streams in each segment. Default is 1.
 
 @item a
-Set the number of output audio streams, that is also the number of video
+Set the number of output audio streams, that is also the number of audio
 streams in each segment. Default is 0.
 
 @item unsafe