doc: fix optional parameters in n-api.md

The thread_finalize_data and thread_finalize_cb parameters in
napi_create_threadsafe_function are optional.

PR-URL: https://github.com/nodejs/node/pull/22998


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>