minor #6162 [Reference] add missing version number (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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[Reference] add missing version number

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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60e1d0d [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::