bug #6334 Fixed yaml configuration of app.exception_controller (AAstakhov)

This PR was squashed before being merged into the 2.3 branch (closes #6334).

Discussion
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Fixed yaml configuration of app.exception_controller

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

Fixed yaml code-block for app.exception_controller service: indented lines with 'class' and 'arguments' configuration to make this example functional.

Commits
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95a3903 Fixed yaml configuration of app.exception_controller

Author: wouterj

cookbook/controller/error_pages.rst

@@ -233,8 +233,8 @@ In that case, you might want to override one or both of the ``showAction()`` and
             # app/config/services.yml
             services:
                 app.exception_controller:
-                class: AppBundle\Controller\CustomExceptionController
-                arguments: ['@twig', '%kernel.debug%']
+                    class: AppBundle\Controller\CustomExceptionController
+                    arguments: ['@twig', '%kernel.debug%']
 
         .. code-block:: xml