Update the instructions for installing with flatpak
A user reported that
flatpak install --user https://flathub.org/repo/appstream/org.pitivi.Pitivi.flatpakref complained:
Required runtime for org.pitivi.Pitivi/x86_64/stable (org.gnome.Platform/x86_64/3.24) is not installed, searching... The required runtime org.gnome.Platform/x86_64/3.24 was not found in a configured remote.
On https://flathub.org/apps/ I see
flatpak install --from https://flathub.org/repo/appstream/org.pitivi.Pitivi.flatpakref ..