Documentation: remove current_security() reference

Commit 15322a0d ("lsm: remove current_security()") removed
current_security() from the sources.

Signed-off-by: Denis Efremov <efremov@linux.com>
Link: https://lore.kernel.org/r/20200830142509.5738-1-efremov@linux.com


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