Slightly clarify expression in for the anullsrc source documentation.

Originally committed as revision 25191 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/filters.texi

@@ -34,8 +34,8 @@ integer or a string representing a channel layout. The default value
 of @var{channel_layout} is 3, which corresponds to CH_LAYOUT_STEREO.
 
 Check the channel_layout_map definition in
-@file{libavcodec/audioconvert.c} for the correspondance between string
-and channel layout values.
+@file{libavcodec/audioconvert.c} for the mapping between strings and
+channel layout values.
 
 Follow some examples:
 @example