minor #7978 [Router] Fix custom router loader service declaration (michaelperrin)

This PR was merged into the 3.3 branch.

Discussion
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[Router] Fix custom router loader service declaration

The service tag was not declared correctly.

Commits
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ae958b8 Fix custom router loader service declaration

Author: xabbuh

routing/custom_route_loader.rst

@@ -147,7 +147,8 @@ Now define a service for the ``ExtraLoader``:
         services:
             # ...
 
-            AppBundle\Routing\ExtraLoader: [routing.loader]
+            AppBundle\Routing\ExtraLoader:
+                tags: [routing.loader]
 
     .. code-block:: xml