If you need or want to replace the PrtScr shortcut do the following:
1.- Release the PrtScr binding by this command
gsettings set org.gnome.settings-daemon.plugins.media-keys screenshot '[]'
2.- Go to Settings -> Devices -> Keyboard and scroll to the end. Press + and you will create custom shortcut.
3.- Enter name: "flameshot", command: /usr/bin/flameshot gui.
4.- Set shortcut to PrtScr (print).
That is it. Next time you push PrtScr flameshot will be launched.