docs: SafeSetID: fix a warning

As reported by Sphinx 2.4.4:

	docs/Documentation/admin-guide/LSM/SafeSetID.rst:110: WARNING: Title underline too short.

	Note on GID policies and setgroups()
	==================

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4afa281c170daabd1ce522653d5d5d5078ebd92c.1603791716.git.mchehab+huawei@kernel.org


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