doc: fix minor typo in n-api.md

Correct parameter name for `napi_is_error` description.

PR-URL: https://github.com/nodejs/node/pull/23310


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>