minor symfony#9268 Fix "RequestExceptionInterface" link (liviubalan)

This PR was submitted for the 3.3 branch but it was merged into the 3.4 branch instead (closes symfony#9268).

Discussion
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Fix "RequestExceptionInterface" link

Change http://api.symfony.com/3.3/Symfony/Component/HttpKernel/Exception/RequestExceptionInterface.html to http://api.symfony.com/3.3/Symfony/Component/HttpFoundation/Exception/RequestExceptionInterface.html

Commits
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a693dd4 Fix "RequestExceptionInterface" link

Author: javiereguiluz

components/http_kernel.rst

@@ -579,7 +579,7 @@ below for more details).
        :class:`Symfony\\Component\\HttpKernel\\Exception\\HttpException` class.
 
     3) If the original exception implements
-       :class:`Symfony\\Component\\HttpKernel\\Exception\\RequestExceptionInterface`,
+       :class:`Symfony\\Component\\HttpFoundation\\Exception\\RequestExceptionInterface`,
        then the status code of the ``FlattenException`` object is populated with
        ``400`` and no other headers are modified.