Added a missing reference to fix doc build

Author: javiereguiluz

components/lock.rst

@@ -573,3 +573,4 @@ are still running.
 .. _Packagist: https://packagist.org/packages/symfony/lock
 .. _`PHP semaphore functions`: http://php.net/manual/en/book.sem.php
 .. _`PDO`: https://php.net/pdo
+.. _`Doctrine DBAL Connection`: https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Connection.php