meta: align email address in README/.mailmap/AUTHORS

Modify .mailmap so Myles has the same email address in AUTHORS as he
does in README.

PR-URL: https://github.com/nodejs/node/pull/39503


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>