Fix Hamiltonian form in Kepler problem example

[abhro]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• Any new documentation only uses public API

Additional context

The Hamiltonian at any point should not depend on the generalized velocity, but instead the momentum. For this problem it doesn't make as much difference, but users can get confused about the equations of motion if the Hamiltonian already includes the generalized velocities.