Fix build warning

Replace Gtk.show_uri with Gtk.show_uri_on_window as suggested in
https://valadoc.org/gtk+-3.0/Gtk.show_uri.html
4 jobs for master in 6 minutes and 37 seconds
Status Job ID Name Coverage
  Test
passed #940331
docs

00:05:03

passed #940329
flatpak
flatpak

00:04:30

passed #940330
lint

00:00:10

 
  Deploy
passed #940332
nightly

00:01:34