cgroups: Documentation: fix wrong cgroupfs paths

Few paths used as example to describe cgroupfs usage have been wrong
from f6e07d38 ("Documentation: update cgroupfs mount point") by
mistake. This patch fix those trivial wrong paths.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>