Broken link in InputFieldCKEditor README.md

[gRegorLove]

Short description of the issue

The PW admin links to these instructions and the "ready-to-go mystyles.js" link there is 404.

Steps to reproduce the issue

1. Edit a textarea field like "body"
2. Select the "Input" tab
3. Scroll to "Custom Editor JS Styles Set"
4. Click the "Instructions" link in the field's description text
5. Click the "ready-to-go-mystyles.js" link

Setup/Environment

• ProcessWire version: 3.0.200