README example: explicit interface/path/service for Notifications example

While it's maybe handy that #[dbus_proxy] defaults to org.freedesktop.NameOfTrait etc., it is very perplexing for someone new to zbus to understand how it can magically address the correct service without supplying a service, interface and path anywhere.

So I propose to provide the arguments to dbus_proxy explicitly in the README example, or at least explain the defaults.