1: agree to sell the item to the customer at the incorrectly advertised price, or...
2: withdraw the item from sale, then reprice the item correctly and make it available for sale at the corrected price (i.e. re-shelve it with a new price sticker)
That's what the article above said, as being the national standard now... so people can't complain or demand that they have to sell an item at the lower price if the store doesn't want to (as per the second option stores have).