use literals for external class names

The `class` role does not work for classes whose API documentation is
not rendered on symfony.com.

Author: xabbuh

reference/configuration/framework.rst

@@ -1482,7 +1482,7 @@ cache
 **type**: ``string``
 
 The service that is used to persist class metadata in a cache. The service
-has to implement the :class:`Doctrine\\Common\\Cache\\Cache` interface.
+has to implement the ``Doctrine\Common\Cache\Cache`` interface.
 
 .. seealso::