Fix typo.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15034 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/bitstream.h

@@ -364,7 +364,7 @@ static inline void set_put_bits_buffer_size(PutBitContext *s, int size){
 
 /* Bitstream reader API docs:
 name
-    abritary name which is used as prefix for the internal variables
+    arbitrary name which is used as prefix for the internal variables
 
 gb
     getbitcontext