Fix box2d example
Fix box2d example

Use contact object instead of separete variables

Signed-off-by: Roger Zanoni <d89d7f2abb4ad608dc5b9534d7ccd45e20963c5f@openbossa.org>

kenvandine/Bacon2D,kenvandine/Bacon2D,arcrowel/Bacon2D,paulovap/Bacon2D,arcrowel/Bacon2D,paulovap/Bacon2D