minor #4697 Set twig service as private (ifdattic)

This PR was merged into the 2.3 branch.

Discussion
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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 (for performance reasons)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets |

Commits
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7427051 Set twig service as private

Author: weaverryan

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 }