docs: broken links and typos

[SethFalco]

This is a minor contribution while I was working on making some badges.

Fixes a minor typo, it's "GitHub" not "Github".

In the API documentation the README is displayed.
However, relative links are kept relative, so they break as they should be linking to github.com, not contributing.shields.io.

This is resolved by using absolute URLs instead of relative URLs.

[shields-ci]

	Messages
📖	Thanks for contributing to our documentation. We ❤️ our documentarians!
📖	✨ Thanks for your contribution to Shields, @SethFalco!

Generated by 🚫 dangerJS against aa66131

[calebcartwright]

Thanks @SethFalco! The changes seem reasonable at first blush, though I do recall we had to make some tradeoffs with our documentation links because it wasn't possible to have them work everywhere. I'm unable to find the specifics because my GitHub search skills are failing me at the moment, but @chris48s do you recall the details/know if these proposed changes could have any potential unintended side effects?

[chris48s]

#4139 still stands but there is nothing here that makes anything worse.
It is on my "if I suddenly have a pile of spare time.." list to have a look at other ways to generate contributing.shields.io but that is.. yet to transpire

[calebcartwright]

LGTM, thanks!