De-Coreify Gitian documentation

Summary:
Gitian Build documentation still contained references to Bitcoin Core.
This changes them to Bitcoin ABC

Test Plan: read the document

Reviewers: schancel, matiu, jasonbcox, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1454