doc: improve module.builtinModules text

`module is maintained by a third-party module` seems confusing. Changed
to `module is maintained by a third-party`.

PR-URL: https://github.com/nodejs/node/pull/17865


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>