doc: fix typo in n-api.md

This commit fixes what I think is a typo, even though the section in
question does talk about a environment (env).

PR-URL: https://github.com/nodejs/node/pull/33319


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>