doc: fix outdated code sample in n-api.md

code samples of napi_create_object and napi_property_descriptor were
not updated to latest API.

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/15581


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>