update test

Note still have double reporting, see issue #13.

Author: peterjc

tests/RST212/short_underline.py

@@ -11,7 +11,7 @@
 and that is considered an error. This should fail::
 
     $ flake8 --select RST RST212/short_underline.py
-    RST212/short_underline.py:10:1: RST299 Title underline too short.
+    RST212/short_underline.py:10:1: RST212 Title underline too short.
 
 Nice
 ====