doc: fix outdated code sample in n-api.md
Code sample of napi_create_object and napi_property_descriptor were not updated to latest API. Ran into this problem while migrating NAN projects to N-API.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc