minor #12393 Update doctrine.rst (hectorprats)

This PR was squashed before being merged into the 4.3 branch (closes #12393).

Discussion
----------

Update doctrine.rst

Uploading code of directly access in controller of the custom dbal connections.

<!--

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
-------

c1b7aec Update doctrine.rst

Author: wouterj

reference/configuration/doctrine.rst

@@ -155,7 +155,11 @@ which is the first one defined or the one configured via the
 ``default_connection`` parameter.
 
 Each connection is also accessible via the ``doctrine.dbal.[name]_connection``
-service where ``[name]`` is the name of the connection.
+service where ``[name]`` is the name of the connection. Or directly in the controller with 
+getConnection method and the name of the connection::
+
+    $connection = $this->getDoctrine()->getConnection('customer');
+    $result = $connection->fetchAll('select name from customer');
 
 Doctrine ORM Configuration
 --------------------------