No action required for now.
This is because torproject deprecated that repository. That was to be expected. The repository we're using now is already preconfigured (in /etc/apt/sources.list.d/torproject.list).
Taking action isn't important at this stage. You can comment it out (by putting a # in front of it) in /etc/apt/sources.list.d/torproject.list if you want. If you don't do it, it will be auto fixed with next upgrade of Whonix.