Set twig service as private

During the best practices workshop it was suggested that it's best to set services to private if they won't be accessed from your code/container

Author: ifdattic

best_practices/templates.rst

@@ -153,6 +153,7 @@ name is irrelevant because you never use it in your own code):
         app.twig.app_extension:
             class:     AppBundle\Twig\AppExtension
             arguments: ["@markdown"]
+            public: false
             tags:
                 - { name: twig.extension }