docs: mm: userfaultfd.rst: use a cross-reference for a section

Instead of using "foo", let's use `foo`_, with is a ReST way of
saying that foo is a section of the document. With that, after
building the docs, an hyperlink is generated.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/f46b45f1aaec233217f2e0b0438bbd8cc16fe17b.1586881715.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/mm/userfaultfd.rst

@@ -79,7 +79,7 @@ or shared memory need to set the corresponding flag in
 If the userland desires to receive notifications for events other than
 page faults, it has to verify that ``uffdio_api.features`` has appropriate
 ``UFFD_FEATURE_EVENT_*`` bits set. These events are described in more
-detail below in "Non-cooperative userfaultfd" section.
+detail below in `Non-cooperative userfaultfd`_ section.
 
 Once the ``userfaultfd`` has been enabled the ``UFFDIO_REGISTER`` ioctl should
 be invoked (if present in the returned ``uffdio_api.ioctls`` bitmask) to