minor #7635 Update installation.rst (atailouloute)

This PR was merged into the 2.7 branch.

Discussion
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Update installation.rst

Improve the display of the configuration example (http://symfony.com/doc/2.7/cookbook/bundles/installation.html#c-configure-the-bundle)

Commits
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a9faf21 Update installation.rst

Author: xabbuh

bundles/installation.rst

@@ -118,7 +118,7 @@ of the bundle's configuration:
 
 The output will look like this:
 
-.. code-block:: text
+.. code-block:: yaml
 
     assetic:
         debug:                '%kernel.debug%'