doc: fix suspicious heading emphasis in n-api.md

PR-URL: https://github.com/nodejs/node/pull/20086


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>