cgroups: remove deprecated subsystem from examples.

The 'ns' cgroup is considered deprecated. Change the cgroup subsystem
used in the examples of the cgroup documentation from 'ns' to 'blkio'.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>