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 <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>