Fix brackets in CONTRIBUTING.md

Summary: Markdown link had the wrong kind of brackets. This fixes them.

Test Plan: Check that the link displays properly in Markdown viewer

Reviewers: deadalnix, schancel, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1475