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

This PR was merged into the 4.3 branch.

Discussion
----------

Use :: instead of code block php

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

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;