docs: fix broken link to GNOME notifications spec (electron#32934)

docs/tutorial/notifications.md

@@ -146,7 +146,7 @@ desktop environment that follows [Desktop Notifications
 Specification][notification-spec], including Cinnamon, Enlightenment, Unity,
 GNOME, KDE.
 
-[notification-spec]: https://developer.gnome.org/notification-spec/
+[notification-spec]: https://developer-old.gnome.org/notification-spec/
 [app-user-model-id]: https://msdn.microsoft.com/en-us/library/windows/desktop/dd378459(v=vs.85).aspx
 [set-app-user-model-id]: ../api/app.md#appsetappusermodelidid-windows
 [squirrel-events]: https://github.com/electron/windows-installer/blob/master/README.md#handling-squirrel-events