Well, I was about to post petty much the same thing as Sally ("I don't know what happened, but I don't think it's bad enough to give up on the plant yet.")
It doesn't look like anything to worry about to me... and if that plant is ahead of the others I would use it, myself. I wouldn't bother pinching the leaf off -- it just looks like a little sunburn or heat stress to me. (My cucumber leaves have some similar spots after wilting a bit during our one and only 90-degree day).
