cpuset: trivial documentation fix s/N_MEMORY/N_HIGH_MEMORY/

Current implementation of cpuset track N_HIGH_MEMORY instead N_MEMORY.
(N_MEMORY doesn't exist in current implementation)

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Paul Jackson <pj@sgi.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>