README example: Uses `examples/utils` instead of `diffcp.utils`

[bamos]

The example in the readme uses diffcp.utils.random_cone_prog which is actually defined here: https://github.com/cvxgrp/diffcp/blob/cbea99715974e36050232d20ff22c38bb92ea91f/examples/utils.py

Perhaps the easiest solution here is to move this inside of diffcp.utils and then add the utils import to __init__ so the example in the README runs?

[bamos]

(Have done this locally and can send in a PR)