fixing reference to deprecated class

Author: weaverryan

components/http_kernel.rst

@@ -548,7 +548,7 @@ below for more details).
     The listener has several goals:
 
     1) The thrown exception is converted into a
-       :class:`Symfony\\Component\\HttpKernel\\Exception\\FlattenException`
+       :class:`Symfony\\Component\\Debug\\Exception\\FlattenException`
        object, which contains all the information about the request, but which
        can be printed and serialized.