The best thing to do is leave it to heal on its own, although if the edges are jagged you can try cleaning them up carefully with a razor so that it heals evenly (don't cut too deep). Sounds like that might not be practical given how the height of the damage, but if you can it should be like a narrow oval that is pointed at the top and bottom. Over time the bark will fill in the spot and cover it just as it would normally cover the area where the branch was removed.

Beware of anyone who offers to "seal" the wound with tar, as is sometimes the practice of landscapers. That would only serve to prolong the healing process and can do more damage in the long term. Some still do it to satisfy their customers despite knowing that it provides no benefit.