cgroup: Trivial correction to reflect controller.

Trivial correction in menuconfig help to reflect PIDs as
controller instead of subsystem to align to rest of the text
and documentation.
Signed-off-by: Parav Pandit <pandit.parav@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

init/Kconfig

@@ -1047,10 +1047,10 @@ config CGROUP_PIDS
 	  is fairly trivial to reach PID exhaustion before you reach even a
 	  conservative kmemcg limit. As a result, it is possible to grind a
 	  system to halt without being limited by other cgroup policies. The
-	  PIDs cgroup subsystem is designed to stop this from happening.
+	  PIDs controller is designed to stop this from happening.
 
 	  It should be noted that organisational operations (such as attaching
-	  to a cgroup hierarchy will *not* be blocked by the PIDs subsystem),
+	  to a cgroup hierarchy will *not* be blocked by the PIDs controller),
 	  since the PIDs limit only affects a process's ability to fork, not to
 	  attach to a cgroup.