meta: align collaborator name in .mailmap/AUTHORS with README

I exchanged email with Gerhard to make sure that this was the rendering
of his name that was correct.

PR-URL: https://github.com/nodejs/node/pull/39489


Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>