Add missing "s" from format string.

This closes SourceForge patch #101714.

Lib/mailbox.py

@@ -272,7 +272,7 @@ def _test():
             f = msg.getheader('from') or ""
             s = msg.getheader('subject') or ""
             d = msg.getheader('date') or ""
-            print '-%20.20s   %20.20   %-30.30s'%(f, d[5:], s)
+            print '-%20.20s   %20.20s   %-30.30s'%(f, d[5:], s)
 
 
 if __name__ == '__main__':