Tour de Source is missing (404 Not Found)

[benhovinga]

The Tour de Source link in CONTRIBUTING.md is broken and returns Error 404 Not Found

.github/CONTRIBUTING.md

Line 15 in b19144b

Before contributing, we recommend you read the [Tour de Source: NextAuth.js](https://sourcegraph.com/notebooks/Tm90ZWJvb2s6MTc2MQ==) post to become more familiar with the libraries inner workings.