Broken link to Github flow

[a-nap]

The challenge "Branches aren't just for birds" contains a link to a guide that has moved from:
guides.github.com/overviews/flow

to here:
https://docs.github.com/en/get-started/quickstart/github-flow.

The former redirects to the documentations page: https://docs.github.com/en

[zcline91]

I don't think the docs.github.com page has the same content as the old guides.github.com page. I believe it was referring to the page https://web.archive.org/web/20210226105433/https://guides.github.com/introduction/flow/.
I've added a pull request to fix the links (#359).