doc/APIchanges: add missing entry for av_format_probe_score() function addition

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2012-10-22
 
 API changes, most recent first:
 
+2013-08-29 - e31db62 - lavf 55.15.100 - avformat.h
+  Add av_format_get_probe_score().
+
 2013-08-xx - xxxxxxx - lsws 2.5.100 -
   Add a sws_dither AVOption, allowing to set the dither algorithm used