minor #4844 Update routing.rst (bglamer)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4844).

Discussion
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Update routing.rst

The first code type of "requirements of route conditions" is marked as PHP but it is annotations.

Commits
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313c1b9 Update routing.rst

Author: wouterj

book/routing.rst

@@ -647,7 +647,7 @@ requirements can easily be added for each parameter. For example:
 
 .. configuration-block::
 
-    .. code-block:: php
+    .. code-block:: php-annotations
 
         // src/AppBundle/Controller/BlogController.php