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>

Documentation/cgroups/cpusets.txt

@@ -180,7 +180,7 @@ files describing that cpuset:
  - cpuset.sched_load_balance flag: if set, load balance within CPUs on that cpuset
  - cpuset.sched_relax_domain_level: the searching range when migrating tasks
 
-In addition, the root cpuset only has the following file:
+In addition, only the root cpuset has the following file:
  - cpuset.memory_pressure_enabled flag: compute memory_pressure?
 
 New cpusets are created using the mkdir system call or shell