AppData: update screenshots
I think the link to the wiki didn't work, the screenshot for the flatpak is missing in gnome-software. The wiki is anyway not a great place to host screenshots for the AppData. Hosting it in the git repo is not great either because it makes the git repo bigger, it takes a longer time to git clone, to run git gc, etc, and the resulting URL is not nice since it requires the git service on the server (it's not just a static file). So I've chosen people.gnome.org, if in the future I'm no longer around the URL can anyway be changed.
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