Bumpers are meant to protect your car. Don't bother much about these scratches. Next time reverse it more carefully. It is very common to have scratches in those areas with tight parking space.
not much damage. it's better to leave it. do get it repaired when the damage is more. bumpers do get scratched here and there when you keep using your car.! since it is not painted it won't be an eye catcher. you can get it polished if it's a sore eye for you.!
- Clean the surface [complete bumper] with water and make it dry.
- Apply dashboard polish in sufficient quantity to whole dashboard.
- leave it for some time to dry out. [may be 5 to 7 minutes]
- wipe it out with a smooth cotton material [preferably a hosiery material].