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>

Author: mchehab

Documentation/admin-guide/LSM/SafeSetID.rst

@@ -107,7 +107,7 @@ for a UID/GID will prevent that UID/GID from obtaining auxiliary setid
 privileges, such as allowing a user to set up user namespace UID/GID mappings.
 
 Note on GID policies and setgroups()
-==================
+====================================
 In v5.9 we are adding support for limiting CAP_SETGID privileges as was done
 previously for CAP_SETUID. However, for compatibility with common sandboxing
 related code conventions in userspace, we currently allow arbitrary