use literal for class name

Author: xabbuh

components/http_foundation/introduction.rst

@@ -282,7 +282,7 @@ Overriding the Request
     The :method:`Symfony\\Component\\HttpFoundation\\Request::setFactory`
     method was added in Symfony 2.4.
 
-The Request class should not be overridden as it is a data object that
+The ``Request`` class should not be overridden as it is a data object that
 represents an HTTP message. But when moving from a legacy system, adding
 methods or changing some default behavior might help. In that case, register a
 PHP callable that is able to create an instance of your ``Request`` class::