De-Coreify /doc/reduce-traffic.md

Summary:
Change bitcoin-core to bitcoin-abc in /doc/reduce-traffic.md

Part of T59

Test Plan: N/A

Reviewers: deadalnix, freetrader, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D315