It's hard to tell from the pic what it might be. If it is a cut, if there is no swelling, discharge and/or pus, irritation, eye bulging, and there is no cloudiness in the eye itself, then the skin may just heal on it's own. But if you see any of those and/or the eye worsens, or you are concerned about it, have it seen by a vet right away. If it is a cut, the eye itself could have been scratched, so may be a good idea to just take your bun in to get it checked out anyways.