Fix typo in libavcodec/opt.h.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@13912 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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;