De-Coreify /doc/benchmarking.md

Summary:
Change Core to ABC in /doc/benchmarking.md

Part of task 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/D313