minor #9420 Fix FQCN in PHP example for expression language (hacfi)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes #9420).

Discussion
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Fix FQCN in PHP example for expression language

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I was following the example and got an error `You have requested a non-existent service \"AppMailMailerConfiguration\"`. I debugged it and I think it’s not an code issue but just the documentation. The YAML version also uses 4 `\`. Maybe we need to fix the XML version as well but I wasn’t sure and didn’t have time to test it.

Commits
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a268135 Fix FQCN in PHP example for expression language

Author: javiereguiluz

service_container/expression_language.rst

@@ -60,7 +60,7 @@ to another service: ``AppBundle\Mailer``. One way to do this is with an expressi
         $container->autowire(MailerConfiguration::class);
 
         $container->autowire(Mailer::class)
-            ->addArgument(new Expression('service("AppBundle\Mail\MailerConfiguration").getMailerMethod()'));
+            ->addArgument(new Expression('service("AppBundle\\\\Mail\\\\MailerConfiguration").getMailerMethod()'));
 
 To learn more about the expression language syntax, see :doc:`/components/expression_language/syntax`.