lxc.container.conf(5): update guidance for lxc.mount.entry mount target

For years it has been best practice to use a relative path as
the mount target.  But the manpage hasn't reflect that.  Fix it.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Dwight Engen <dwight.engen@oracle.com>