update readme

[jpdeleon]

resolves #350

Description

Simply update the example in README to reflect correct out-of-sample prediction using prophet, and use it for custom backtest

Checklist

• [x] I am making a pull request from a branch other than master
• [x] I have read the CONTRIBUTING.md
• [ ] I have added/edited documentation in a relevant docs/docusaurus markdown file
• [ ] (For new docs, check if not applicable) I have added the id of a new docs md to the docs/docusaurus/sidebars.js file