Fix autoescape_service example

As @fabpot said in symfony/symfony#8833, the `autoescape_service` value must not start with an `@`.

Author: quentin-st

reference/configuration/twig.rst

@@ -41,7 +41,7 @@ TwigBundle Configuration ("twig")
 
             # The following were added in Symfony 2.3.
             # See http://twig.sensiolabs.org/doc/recipes.html#using-the-template-name-to-set-the-default-escaping-strategy
-            autoescape_service:        ~ # Example: '@my_service'
+            autoescape_service:        ~ # Example: 'my_service'
             autoescape_service_method: ~ # use in combination with autoescape_service option
             base_template_class:       ~ # Example: Twig_Template
             cache:                     '%kernel.cache_dir%/twig'