minor symfony#12138 Use :: instead of code block php (OskarStark)

This PR was merged into the 4.3 branch.

Discussion
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Use :: instead of code block php

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Commits
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99a9d9c Use :: instead of code block php

Author: OskarStark

components/mailer.rst

@@ -63,9 +63,7 @@ it:
 
     $ composer require symfony/google-mailer
 
-Then, use the SMTP Gmail transport:
-
-.. code-block:: php
+Then, use the SMTP Gmail transport::
 
     use Symfony\Component\Mailer\Bridge\Google\Smtp\GmailTransport;