meta: update email address for collaborator

In email, Saúl indicated that they've switched to using their
personalized domain email address.  Update their email in README
accordingly. This will align their email in the README with their email
in .mailmap, which will in turn assist the accuracy of tools for
contributor metrics.

PR-URL: https://github.com/nodejs/node/pull/39511


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>