LSM: fix documentation for the task_setscheduler hook

The task_setscheduler hook was changed in the commit
"security: remove unused parameter from security_task_setscheduler()"
(b0ae1981

). The arguments @policy, @lp were removed from the hook.
This patch updates the documentation accordingly.

Signed-off-by: Denis Efremov <efremov@ispras.ru>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.morris@microsoft.com>