You probably don’t want/need to do anything. Anything you do is just going to crack again anyways. If you want it right (there is no point to doing this) you’d actually want to open the crack even more and put expansion joint in there. The stoop and pad will always move differently. This will crack again every spring if you use anything hard. You could clean it out and use polymeric sand (the stuff for patio joints) to fill it in if it really bothers you. That would stay in the crack and remain flexible, or use a caulk for concrete repair as the other guy recommended.