minor #6237 use literals for external class names (xabbuh)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #6237).

Discussion
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use literals for external class names

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

Commits
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0baa317 use literals for external class names

Author: wouterj

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::