Rename seed for bitcoinforks.org

Summary: It appears to have been renamed.

Test Plan:
The following returns results:

dig seed-bch.bitcoinforks.org
dig testnet-seed-bch.bitcoinforks.org

While the following do not:

dig seed-abc.bitcoinforks.org
dig testnet-seed-abc.bitcoinforks.org

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7188