Small fix in CONTRIBUTING.md - clang-format-7 still mentioned, but 8 is required

Summary: fix outdated mention of clang-format-7

Test Plan: note that arc lint fails unless clang-format-8 is installed, even if clang-format-7 is available

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

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

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D4634