plex: fix preStart permissions

nixos/modules/services/misc/plex.nix

···
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
       preStart = ''
-        test -d "${cfg.dataDir}" || {
+        test -d "${cfg.dataDir}/Plex Media Server" || {
           echo "Creating initial Plex data directory in \"${cfg.dataDir}\"."
           mkdir -p "${cfg.dataDir}/Plex Media Server"
           chown -R ${cfg.user}:${cfg.group} "${cfg.dataDir}"