Fix dead link in CONTRIBUTING.md

[jakeboone02]

What:

Updated link to KCD's PR course on Egghead.io.

Why:

Existing link leads to 404 page.

Checklist:

• Ready to be merged

I'm wondering if it should be http://kcd.im/pull-request instead of the direct link.

Also, the same bad link appears on the testing-library/jest-dom contributing page, too:

https://github.com/testing-library/jest-dom/blob/main/CONTRIBUTING.md

I can submit a similar PR for that repo if necessary.

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