doc: update collaborator email address

Per email conversation with Shigeki Ohtsu, updating email address in
docs. The current listed email address does not work anymore.

PR-URL: https://github.com/nodejs/node/pull/11996


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@ohtsu.org>