Update README.md

[AtomicRobotMan0101]

trivial edit to add flatpak PrtSc button instructions for Ubuntu

[mmahmoudian]

Thanks for your kind contribution. Two questions though:

1. Is there any particular reason that the full path for flatpak is mentioned? I believe this is not universal path on linux distros. For instance on Guix and Nix the flatpak binary is not in usr/bin/.
2. Are the --branch=stable --arch=x86_64 necessary?