all manner of folk follow different rules for writing it out. you can either add just the apostrophe or add the 's. both are basically correct.
but i do pronounce that extra -s like googa and charlie when i say it.
I add the 's' when I'm writing but I've come across a lot of people who think that's WRONG, and can get quite cross about it.
i guess it depends on where you are, doesn't it?
.. although i can tell you right now, there are violent hordes of people on either side of the issyew, and they get in bloody brawls about it all the time.
if you wanna spend a miserable hour, just explore online. for anybody on one side of the issyew, i can find ten more on the other side.
i am talking about grammarians, by the way, mwhahaha. a hardcore lot.
more than twenty years ago, when i was an english teacher? they were fighting about it then. even in america alone, there'd be one right way to do it on one campus, and the other right way to do it on another campus a thousand miles away.
i could name a hundred grammar issyews that follow this same suit. we won't even get into split infinitives... starting a sentence with a coordinating conjunction... comma splices... the list is very long. even a good many of those rules have gone by the wayside over time.
that's why i say oh to hell with it, just pick a side and stick with it.
if you are a teacher or editor or some such, you can find out what is considered acceptable where you are.
otherwise, if it's yer own family name, you can do whatever the fucque you want.