lavf/avformat.h: fix a few typos in AVFormatContext:avoid_negative_ts doxy

libavformat/avformat.h

@@ -1167,11 +1167,11 @@ typedef struct AVFormatContext {
     int use_wallclock_as_timestamps;
 
     /**
-     * Avoids negative timestamps during muxing
+     * Avoid negative timestamps during muxing.
      *  0 -> allow negative timestamps
      *  1 -> avoid negative timestamps
      * -1 -> choose automatically (default)
-     * Note, this is only works when interleave_packet_per_dts is in use
+     * Note, this only works when interleave_packet_per_dts is in use.
      * - encoding: Set by user via AVOptions (NO direct access)
      * - decoding: unused
      */