Clarify changes file regarding which bugs get fixed.

changes/bug5891

@@ -1,5 +1,5 @@
   o Minor bugfixes:
     - Fix a bug where a bridge authority crashes if it has seen no
       directory requests when it's time to write statistics to disk.
-      Fixes #5508 and #5891.
+      Fixes bug 5891.  Also fixes bug 5508 in a better way.