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

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.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>