[DOC] Update gitian instructions with the new OSX SDK version

Summary: This is a leftover from the SDK update done in D7737.

Test Plan:
Read the doc and check it is compliant with the instructions from
contrib/macdeploy/README.md

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7905