Change 'bugfix on' line for changes/bug1921.

Karsten says: "the ChangeLog should say it's a bugfix on
0.2.2.15-alpha, because enabling stats while Tor is running (which
leads to this false log message) is only possible since then."

Sounds right enough to me.  Tell me if I'm wrong.

changes/bug1921

@@ -1,4 +1,4 @@
   o Minor bugfixes:
     - Demote a warn to info that happens when the CellStatistics option
-      was just enabled. Fixes bug 1921; bugfix on 0.2.2.1-alpha. Reported
+      was just enabled. Fixes bug 1921; bugfix on 0.2.2.15-alpha. Reported
       by Moritz Bartl.