doc/examples/flatpak-autotools: remove references to sdk.gnome.org

Abderrahim Kitounirequested to merge
abderrahim/no-sdk-gnome-org-2 into master

Description

Update thhe flatpak-autotools example to use flathub instead of sdk.gnome.org (which no longer exists).

This merge request, when approved, will close: #690 (closed)

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