Fix warning about unknown escape sequences

systemd warns about:

Ignoring unknown escape sequences: "/nix/store/8f0l1w9g7iv2gz63xzsxfl66ri1cfbkl-plex-1.9.5.4339/usr/lib/plexmediaserver/Plex\ Media\ Server

From a discussion on the forums it seems the 'sh -c' is not needed:

https://forums.plex.tv/discussion/216757/ubuntu-16-04-executable-path-contains-special-characters-error-with-systemd