Update custom constraint example.
Update custom constraint example.

Gjacquenot/simbody,chrisdembia/simbody,elkingtonmcb/simbody,robojukie/simbody,Gjacquenot/simbody,Gjacquenot/simbody,scamille/simbody,elkingtonmcb/simbody,scamille/simbody,robojukie/simbody,chrisdembia/simbody,elkingtonmcb/simbody,simbody/simbody,elen4/simbody,chrisdembia/simbody,scamille/simbody,robojukie/simbody,simbody/simbody,elen4/simbody