[Reference] add missing version number

Author: xabbuh

reference/configuration/debug.rst

@@ -10,7 +10,7 @@ Symfony full-stack framework and can be configured under the ``debug`` key
 in your application configuration. When using XML, you must use the
 ``http://symfony.com/schema/dic/debug`` namespace.
 
-.. versionadded::
+.. versionadded:: 2.6
     The DebugBundle was introduced in Symfony 2.6.
 
 .. tip::