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 <[e-mail]>
Link: https://lore.kernel.org/r/4afa28[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/admin-guide/LSM/SafeSetID.rst

···
 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