MAINTAINERS: update cgroup's document path

cgroup's document path is changed to "cgroup-v1".  update it.

Link: http://lkml.kernel.org/r/1470322507-5161-1-git-send-email-iamyooon@gmail.com


Signed-off-by: seokhoon.yoon <iamyooon@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>