avcodec: Fix successfull typo

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avcodec.h

@@ -6012,7 +6012,7 @@ int av_bsf_list_append2(AVBSFList *lst, const char * bsf_name, AVDictionary **op
  * This function will transform @ref AVBSFList to single @ref AVBSFContext,
  * so the whole chain of bitstream filters can be treated as single filter
  * freshly allocated by av_bsf_alloc().
- * If the call is successfull, @ref AVBSFList structure is freed and lst
+ * If the call is successful, @ref AVBSFList structure is freed and lst
  * will be set to NULL. In case of failure, caller is responsible for
  * freeing the structure by av_bsf_list_free()
  *