[CONTRIBUTING] pin version for eslint

Summary: eslint requires different config files than the eslintrc files currently used. Separately, we should upgrade eslint and standardize for all the JS repos. But for now, at least correct the documentation to prevent recurring errors from users attempting to make diffs.

Test Plan: Proofread

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16449