av_alloc_format_context has been renamed, update AVFormatContext av_class

documentation accordingly.
Patch by avcoder: `gmail_address(ffmpeg)`

Originally committed as revision 17430 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avformat.h

@@ -535,7 +535,7 @@ typedef struct AVChapter {
  * sizeof(AVFormatContext) must not be used outside libav*.
  */
 typedef struct AVFormatContext {
-    const AVClass *av_class; /**< Set by av_alloc_format_context. */
+    const AVClass *av_class; /**< Set by avformat_alloc_context. */
     /* Can only be iformat or oformat, not both at the same time. */
     struct AVInputFormat *iformat;
     struct AVOutputFormat *oformat;