Bug#869277: /usr/bin/notify-send: Improve the manpage

[Pablo A B]

Now not even that link works. Galago Project doesn't exist anymore.
Last archive.org snapshot:
https://web.archive.org/web/20220422104829/https://www.galago-project.org/specs/notification/
Maybe should be replaced with
https://specifications.freedesktop.org/notification-spec/notification-spec-latest.html
as mention [here](https://gitlab.gnome.org/GNOME/libnotify/-/blob/master/README.md)