doc: reword possessive form of Node.js in adding-new-napi-api.md

Throughout the docs, we sometimes write the possessive of _Node.js_ as
_Node.js'_ and other times as _Node.js's_. The former conforms with some
generally accepted style guides (e.g., Associated Press Stylebook) while
the latter complies with others (e.g., Chicago Manual of Style).

Since there is no clear authoritative answer as to which form is
correct, and since (at least to me) both are visually jarring and
sometimes cause a pause to understand, I'd like to reword things to
eliminate the possessive form where possible.

This is one of those examples.

PR-URL: https://github.com/nodejs/node/pull/31748
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/guides/adding-new-napi-api.md

@@ -1,6 +1,6 @@
 # Contributing a new API to N-API
 
-N-API is Node.js's next generation ABI-stable API for native modules.
+N-API is the next-generation ABI-stable API for native modules.
 While improving the API surface is encouraged and welcomed, the following are
 a set of principles and guidelines to keep in mind while adding a new
 N-API API.