APIchanges: fill-in git commit hash for av_get_bytes_per_sample() addition

doc/APIchanges

@@ -13,7 +13,7 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
-2011-06-07 - xxxxxxx - lavu 51.4.0 - av_get_bytes_per_sample()
+2011-06-07 - a6703fa - lavu 51.4.0 - av_get_bytes_per_sample()
   Add av_get_bytes_per_sample() in libavutil/samplefmt.h.
   Deprecate av_get_bits_per_sample_fmt().