minor symfony#4500 Link to standard edition (harikt)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#4500).

Discussion
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Link to standard edition

| Q             | A
| ------------- | ---
| Doc fix?      | [yes]
| New docs?     | [no]
| Applies to    | [all]
| Fixed tickets | [N/A]

so users can get the app/AppKernel.php if needed.

Commits
-------

030d7d6 Link to standard edition so users can get the app/AppKernel.php if needed.

Author: weaverryan

book/from_flat_php_to_symfony2.rst

@@ -646,7 +646,7 @@ A routing configuration map provides this information in a readable format:
 
 Now that Symfony is handling all the mundane tasks, the front controller
 is dead simple. And since it does so little, you'll never have to touch
-it once it's created (and if you use a Symfony distribution, you won't
+it once it's created (and if you use a `Symfony distribution`_, you won't
 even need to create it!)::
 
     // web/app.php
@@ -761,3 +761,4 @@ Learn more from the Cookbook
 .. _`Twig`: http://twig.sensiolabs.org
 .. _`Varnish`: https://www.varnish-cache.org/
 .. _`PHPUnit`: http://www.phpunit.de
+.. _`Symfony distribution`: https://github.com/symfony/symfony-standard