Trivial: Corrected comment array name from pnSeeds6 to pnSeed6

Summary: Backport of Bitcoin Core PR12986 https://github.com/bitcoin/bitcoin/pull/12986

Test Plan: cmake --build ./build --config Release --target check-functional -- -j 6

Reviewers: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc, deadalnix, nakihito

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc, nakihito

Subscribers: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D4553