Make links clickable in CONTRIBUTING.md

Summary: Remove inline code quotes, which make the links non-clickable in Github.

Test Plan: N/A

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2178