Then flag it and move on. From the site guidelines:

Please don't submit comments complaining that a submission is inappropriate for the site. If you think something is spam or offtopic, flag it by going to its page and clicking on the "flag" link. (Not all users will see this; there is a karma threshold.) If you flag something, please don't also comment that you did.

Yeah, it matches up with how Americans pronounce it, too. British English speakers (like me) pronounce it 'ren AY sonce', while Americans pronounce it 'ren uh sonce'.

Etymologically, the word comes from the French for rebirth, which is spelled 'renaissance'. Normally, it's prudent to take a live-and-let-live approach to the development of language as that's how language evolves, but in this instance the Americanised spelling is downright barbarous and not holding with the origins or spirit of the word. In this case, I'd argue that the American spelling isn't different in the way color or aluminum are, it's just wrong.

Growing up in the US in the '70s and '80s, I'd never seen it this way, either, and a little googling shows that it's a very obscure variant, with fewer than 60K uses, compared to renaissance with over 13M uses. I think someone saw it mentioned as an American variant and became confused into thinking that it was a common American variant.