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

nixos/modules/services/misc/plex.nix

···
         User = cfg.user;
         Group = cfg.group;
         PermissionsStartOnly = "true";
-        ExecStart = "/bin/sh -c ${cfg.package}/usr/lib/plexmediaserver/Plex\\ Media\\ Server";
+        ExecStart = "\"${cfg.package}/usr/lib/plexmediaserver/Plex Media Server\"";
         KillSignal = "SIGQUIT";
         Restart = "on-failure";
       };