Pretty normal, this is where the top rubs on the rubber on the boot lid on the fold line when retracting. Mine has it also on both sides super annoying.
Good news its aesthetic and there is a sealed layer beneath. In all honesty there isn't a good way of repairing it, There are kits, you can glue it but it just makes it look worse from an aesthetic point of view. Basically highlights it more. There are plenty of threads about this issue give it a search
Only way to solve properly is a replacement. Either one from a breaker which cost about £200 or a new one which is about £800 both without fitting.