Satisfy grammatical nits.

Originally committed as revision 20910 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -13,13 +13,13 @@ libavutil:   2009-03-08
 API changes, most recent first:
 
 2009-12-13 - r20834 - lavf 52.43.0 - metadata API
-  add av_metadata_set2(), AV_METADATA_DONT_STRDUP_KEY and AV_METADATA_DONT_STRDUP_VAL
+  Add av_metadata_set2(), AV_METADATA_DONT_STRDUP_KEY and AV_METADATA_DONT_STRDUP_VAL.
 
 2009-12-13 - r20829 - lavu 50.7.0 - avstring.h API
-  add av_d2str()
+  Add av_d2str().
 
 2009-12-13 - r20826 - lavc 52.42.0 - AVStream
-  add avg_frame_rate
+  Add avg_frame_rate.
 
 2009-12-12 - r20808 - lavu 50.6.0 - av_bmg_next()
   Introduce the av_bmg_next() function.