avio: update avio_alloc_context() doxy

It must be freed using avio_context_free() starting with commit
99684f3a.
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/avio.h

@@ -195,7 +195,7 @@ int avio_check(const char *url, int flags);
 
 /**
  * Allocate and initialize an AVIOContext for buffered I/O. It must be later
- * freed with av_free().
+ * freed with avio_context_free().
  *
  * @param buffer Memory block for input/output operations via AVIOContext.
  *        The buffer must be allocated with av_malloc() and friends.