trivial: Give the right path in Documentation example

While the Documentation example creates /cgroup/test, it removes
/test/cgroup, which is clearly not the intended path. Change that to
/cgroup/test.

Acked-by: KAMEZAWA Hiroyuki <[e-mail]>
Signed-off-by: Thadeu Lima de Souza Cascardo <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

Documentation/cgroups/memcg_test.txt

···
 	(Shell-B)
 	# move all tasks in /cgroup/test to /cgroup
 	# /sbin/swapoff -a
-	# rmdir /test/cgroup
+	# rmdir /cgroup/test
 	# kill malloc task.
 
 	Of course, tmpfs v.s. swapoff test should be tested, too.