doc: correct JavaScript primitive value names in n-api.md

PR-URL: https://github.com/nodejs/node/pull/39129


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>