minor #1426 Specify relation of twig-bridge to symfony/config (frost-nzcr4)

This PR was submitted for the master branch but it was merged into the 1.3 branch instead (closes #1426).

Discussion
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Specify relation of twig-bridge to symfony/config

When I install `symfony/twig-bridge` I've got `Class 'Symfony\Component\Config\Util\XmlUtils' not found in /vendor/symfony/translation/Loader/XliffFileLoader.php on line 53`

It's kinda related to comment https://github.com/symfony/symfony/issues/13194#issuecomment-90893304

Commits
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e2e417fa Specify relation of twig-bridge to symfony/config

doc/providers/form.rst

@@ -73,7 +73,7 @@ Registering
 
     .. code-block:: bash
 
-        composer require symfony/twig-bridge symfony/translation
+        composer require symfony/twig-bridge symfony/config symfony/translation
 
 Usage
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