Update LXC documentation

Removed alternative location from example since AppArmor on Apertis
prevents mounts for LXC outside of the default location.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7417