that could be a lot of things, sunburn, the leaf got poked or scratched and it got a little mark... hard to say. what to really watch out for is if the spots continue to grow larger, or appear wet looking, like bruised and get bigger, as that is a bacterial or fungal rot that will continue to spread. but i think you just have a small blemish, and probably nothing to really worry over. it happens. it happens a lot really. but if you post a better close up of the spot, people might be able to help more on this one.