User double \ instead of four \ in expression service

Author: jbdelhommeau

service_container/expression_language.rst

@@ -28,7 +28,7 @@ to another service: ``App\Mailer``. One way to do this is with an expression:
             App\Mail\MailerConfiguration: ~
 
             App\Mailer:
-                arguments: ["@=service('App\\\\Mail\\\\MailerConfiguration').getMailerMethod()"]
+                arguments: ["@=service('App\\Mail\\MailerConfiguration').getMailerMethod()"]
 
     .. code-block:: xml