minor symfony#6842 Fixed service name (jeremyFreeAgent)

This PR was merged into the 2.7 branch.

Discussion
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Fixed service name

The name was `mailer` in the PHP configuration but it is `app.mailer` elsewhere.

Commits
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3d241a5 Fixed service name

Author: wouterj

service_container/service_decoration.rst

@@ -40,11 +40,11 @@ the original service is lost:
 
     .. code-block:: php
 
-        $container->register('mailer', 'AppBundle\Mailer');
+        $container->register('app.mailer', 'AppBundle\Mailer');
 
         // this replaces the old app.mailer definition with the new one, the
         // old definition is lost
-        $container->register('mailer', 'AppBundle\DecoratingMailer');
+        $container->register('app.mailer', 'AppBundle\DecoratingMailer');
 
 Most of the time, that's exactly what you want to do. But sometimes,
 you might want to decorate the old one instead. In this case, the