nix/modules: fix appview systemd service name

Signed-off-by: Anirudh Oppiliappan <[e-mail]>

nix/modules/appview.nix

···
     };
 
     config = mkIf cfg.enable {
-      systemd.services.tangled.appview = {
+      systemd.services.appview = {
         description = "tangled appview service";
         wantedBy = ["multi-user.target"];