clarify start/end timebase

Originally committed as revision 13254 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avformat.h

@@ -392,7 +392,7 @@ typedef struct AVProgram {
 typedef struct AVChapter {
     int id;                 ///< Unique id to identify the chapter
     AVRational time_base;   ///< Timebase in which the start/end timestamps are specified
-    int64_t start, end;     ///< chapter start/end time in AV_TIME_BASE units
+    int64_t start, end;     ///< chapter start/end time in time_base units
     char *title;            ///< chapter title
 } AVChapter;