I wouldn't rectify this at all.
You basically tell them that you had one year more experience than they thought, but on the other hand you are not even focused enough to get important data on your CV right. They probably didn't take notice of this employment time at all, because it would have meant a one year employment gap which you would have been asked about, so overall this will look negative for you.
Since nobody can accuse you of cheating (the information shown was less good than the correct information), I wouldn't say anything. If you have an offer, either nobody will care, or will care about you making mistakes in important documents.
PS. Nobody will increase your pay after they made an offer. One year experience vs. so unfocused that a person cant even get their CV right - at best this cancels out. It definitely doesnt make you look good.