Broken link in CONTRIBUTING.md

[rbstrachan]

The very last line of CONTRIBUTING.md contains a link that is supposed to redirect to Richard Litt's Guide on amending commits. The link is incorrect (broken) and leads to a 404 Error.

---

The link is currently:

github.com/RichardLitt/docs/blob/master/amending-a-commit-guide.md

The link should be changed to:

github.com/RichardLitt/knowledge/blob/master/github/amending-a-commit-guide.md