Fix typo in continuousPhysics getter and setter

It looks like continuousPhysics was spelled wrong here, but it was getting/setting the correct property on the b2World object. It wasn't clear how to go about testing the changes. For reference, I am on:

* Mountain Lion (10.8.3)
* Xcode 4.6.2 (4H1003)

Let me know if any more changes are required.