Fix git clone instructions

Summary: Reported here: https://github.com/Bitcoin-ABC/bitcoin-abc/issues/255 qshuai didn't want to make the change himself, so I've done it instead.

Test Plan: N/A

Reviewers: deadalnix, schancel, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1796