kernel/cgroup.c: fix 2 kernel-doc warnings

Fix typo and variable name.

tj: Updated @cgrp argument description in cgroup_destroy_css_killed()

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Tejun Heo <tj@kernel.org>