Fix typo in continuousPhysics property/setting

It looks like continuousPhysics was also misspelled here as
in https://github.com/CurveBeryl/Joybox-Box2D/pull/4. The other
vendored files affected in this project, but not changed in
this pull request are:

`vendor/vendor-ios/Box2D.framework/Box2D.framework.bridgesupport`
`vendor/vendor-ios/Box2D.framework/Versions/A/Headers/Dynamics/B2DWorld.h`
`vendor/vendor-osx/box_2d/box_2d.bridgesupport`
`vendor/vendor-osx/box_2d/include/B2DWorld.h`

I assume those will be updated when you pull in the vendored
changes, but let me know if I should update those files as well.