minor #8551 Update YAML service (matt9mg)

This PR was submitted for the master branch but it was merged into the 3.3 branch instead (closes #8551).

Discussion
----------

Update YAML service

Use the new service container configuration in the yaml example

Commits
-------

461c53a Update YAML service

Author: xabbuh

event_dispatcher.rst

@@ -77,8 +77,7 @@ using a special "tag":
 
         # app/config/services.yml
         services:
-            app.exception_listener:
-                class: AppBundle\EventListener\ExceptionListener
+            AppBundle\EventListener\ExceptionListener:
                 tags:
                     - { name: kernel.event_listener, event: kernel.exception }