If it's a very small cut hardly noticeable , you can keep an eye on it to see if it gets any worse. Most healthy fish will heal rapidly if the wound is relatively small. You can always treat it with some antibiotic drops if you see signs of an infection.  The best thing to do is to prevent it from happening again.  If you have sharp objects in the tank such as decorations, you should check to be sure they don't cut or bruise the fish.