The critical shutdown notice string used to have some spaces missing,
which makes it not so pretty.

Add the spaces to satisfy usual English space rules.
Reported-by: Mingcong Bai <jeffbai@aosc.io>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>