Extending mainpage example

I didn't mean "why 3"?
I meant the sign, i.e "why `threshold = background - 3 * sigma` and not `threshold = background + 3 * sigma`?

Threshold is the absolute image level above which to select sources, so it should be higher than `background`, no?

But looking at the detections it obviously works, so we can ignore this for now ... it's just a bit weird.