Edit down ARKitecture docs

[sbenthall]

See the docs:

https://hark.readthedocs.io/en/latest/ARKitecture.html#hark-utilities

"The HARK.utilities module carries a double meaning in its name, as it contains both utility functions (and their derivatives, inverses, and combinations thereof) in the economic modeling sense as well as utilities in the sense of general tools."

This is no longer true as we now have the rewards module.

Actually, this whole larger section of the documentation has lots of material that's not up-to-date. https://github.com/econ-ark/HARK/blob/master/Documentation/ARKitecture.md