Fix typo in changelog

ChangeLog

@@ -25,7 +25,7 @@ Here is the full list of changes/fixes:
   - Remove newline between a ) and , in the beautifier.
   - Add link to automatically open the explain plan on http://explain.depesz.com/
   - Add support to auto_explain, EXPLAIN plan will be added together with top
-    slowest queries whane available in the log file.
+    slowest queries when available in the log file.
   - Add a graph on distributed duration per user. Thanks to Korriliam for the
     patch.
   - Add tree new report: Number of cancelled queries (graph), Queries generating