Yeah this is interesting. It's the Saxon genitive. You have punctuated correctly. I always remember it by something like: 'It is max -his car' (Max's car). You get into disagreement when the noun ends in 's': some say 'my Boss' house' and some 'Boss's house' - on the latter matter there is disagreement, though Boss's house sounds better to me in this case, I don't always follow that rule: 'St. James' Park' for example.'  Sorry I've muddled the punctuation marks a bit....