User double \ instead of four \ in expression service

Author: jbdelhommeau

service_container/expression_language.rst

@@ -29,6 +29,8 @@ to another service: ``App\Mailer``. One way to do this is with an expression:
 
             App\Mailer:
                 arguments: ["@=service('App\\\\Mail\\\\MailerConfiguration').getMailerMethod()"]
+                # Warning: Double escape is necessary with single-quoted strings https://yaml.org/spec/1.2/spec.html#id2787109
+                # arguments: ['@=service("App\\Mail\\MailerConfiguration").getMailerMethod()']
 
     .. code-block:: xml