Fix DSM Postinst script

Minor changes to abide newer DSM OS. Have not verified if this breaks backwards compatibility on the this line change here: https://github.com/ziggimon/plex-media-server-distro-scripts/compare/synology-fix?expand=1#diff-7c2f6bd585e316592076f12ece7025dcR19 I would need confirmation from other source to sign off on it. But it looks reasonably harmless.