landlock: Fix documentation style

It seems that all code should use double backquotes, which is also used
to convert "%" defines.  Let's use an homogeneous style and remove all
use of simple backquotes (which should only be used for emphasis).

Cc: Günther Noack <gnoack3000@gmail.com>
Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220923154207.3311629-4-mic@digikod.net