Fix typo in libavcodec/opt.h.

Originally committed as revision 13912 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/opt.h

@@ -55,7 +55,7 @@ typedef struct AVOption {
 
     /**
      * The offset relative to the context structure where the option
-     * value is stored. It should be 0 for named constant.
+     * value is stored. It should be 0 for named constants.
      */
     int offset;
     enum AVOptionType type;