Fix UAHF references in dnsseed-policy.md

Summary:
Port of Bitcoin Cash Node MR54
https://gitlab.com/bitcoin-cash-node/bitcoin-cash-node/-/merge_requests/54

Test Plan: Read it over, check markdown link.

Reviewers: deadalnix, nakihito, jasonbcox, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5406