I don’t know the name of the file as I don’t use that repo and it’s generated when added through APT. Check in /etc/apt/sources.list.d ( / being the system root, you can go in it with the “Filesystem” shortcut in Caja’s left panel) and you should see a .list file with a name probably like “danielrichter2007-ubuntu-grub-customizer.list”.
When you have the right name, open it with Pluma as admin:
sudo pluma /etc/apt/sources.list.d/danielrichter2007-ubuntu-grub-customizer.list
(Replacing the correct name in the command, obviously.)