Fixed FreeBSD build instructions (by Candunc)

Summary: Documentation fixes for building Bitcoin ABC on FreeBSD (non-GUI version only at the moment)

Test Plan: Verify that the instructions result in a working build on FreeBSD 11.0-RELEASE

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Candunc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: freetrader, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D332