Correct typo.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@8058 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/avcodec.h

@@ -2567,7 +2567,7 @@ void avcodec_flush_buffers(AVCodecContext *avctx);
 
 void avcodec_default_free_buffers(AVCodecContext *s);
 
-/* misc usefull functions */
+/* misc useful functions */
 
 /**
  * returns a single letter to describe the picture type