amend changes file to note that one case is still unfixed

changes/bug21969

@@ -1,3 +1,3 @@
   o Major bugfixes (entry guards):
     - Don't block bootstrapping when a primary bridge is offline and we can't
-      get its descriptor. Fixes bug 21969; bugfix on 0.3.0.3-alpha.
+      get its descriptor. Fixes one case of bug 21969; bugfix on 0.3.0.3-alpha.