docs: security: landlock.rst: avoid using ReST :doc:foo markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9174021ef2c87f395a4cc0895a4b2f7fd97db626.1623824363.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: mchehab

Documentation/security/landlock.rst

@@ -25,7 +25,8 @@ Any user can enforce Landlock rulesets on their processes.  They are merged and
 evaluated according to the inherited ones in a way that ensures that only more
 constraints can be added.
 
-User space documentation can be found here: :doc:`/userspace-api/landlock`.
+User space documentation can be found here:
+Documentation/userspace-api/landlock.rst.
 
 Guiding principles for safe access controls
 ===========================================