fixed backslash escaping

Author: smatyas

components/http_kernel/introduction.rst

@@ -562,7 +562,7 @@ below for more details).
        object. The idea is that these are used in the next step when creating
        the final response. If you want to set custom HTTP headers, you can always
        use the ``setHeaders`` method on exceptions derived from the
-       :class:`Symfony\Component\HttpKernel\Exception\HttpException` class.
+       :class:`Symfony\\Component\\HttpKernel\\Exception\\HttpException` class.
 
     3) A controller is executed and passed the flattened exception. The exact
        controller to render is passed as a constructor argument to this listener.