Update doctrine.rst

Fix for "Title underline too short"

Author: healdropper

reference/configuration/doctrine.rst

@@ -413,7 +413,7 @@ service where ``[name]`` is the name of the connection.
 .. _DBAL documentation: http://docs.doctrine-project.org/projects/doctrine-dbal/en/latest/reference/configuration.html
 
 Shortened Configuration syntax
------------------------------
+------------------------------
 
 When you are only using one entity manager, all config options available
 for each entity manager can be placed directly under ``doctrine.orm`` config