Reference bug number in changes file

changes/bug13941

@@ -1,4 +1,6 @@
   o Minor bugfixes (hidden services):
     - When adding a new hidden-service (for example, via SETCONF) Tor
-      no longer logs a congratulations for running a relay.
+      no longer logs a congratulations for running a relay. Fixes bug
+      13941; bugfix on 0.2.6.1-alpha.
+