APIChanges: document git revision for CODEC_CAP_SLICE_THREADS addition.

doc/APIchanges

@@ -13,7 +13,7 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
-2011-04-21 - XXXXXX - lavc 53.1.0 - avcodec.h
+2011-04-21 - 94f7451 - lavc 53.1.0 - avcodec.h
   Add CODEC_CAP_SLICE_THREADS for codecs supporting sliced threading.
 
 2011-04-15 - lavc 52.120.0 - avcodec.h