[doc] Fix link to article in CONTRIBUTING.md

Summary: The link was dead.

Test Plan:
Tested new link, it works.
https://github.com/Mengerian/bitcoin-abc/blob/article-link/CONTRIBUTING.md

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10484