Documentation: fix ambigous text for root cpuset

Only the root cpuset has cpuset.memory_pressure_enabled flag,
but not the only one.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>