trivial: correct systemd motd directory variable
trivial: correct systemd motd directory variable

fixes the blank variable in systemd unit

fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd