It's its?

For those of you who, like me, twitch a little each time you see it's used instead of its or, less commonly, its used instead of it's, no less than a former president says, "So what?"

I spotted this at his house when I visited last week:

A sign at Monticello, home of Thomas Jefferson

So we should just get over ourselves already.

But I'll probably still twitch a bit.