doc: fix stability text for n-api

While some places list n-api as stable, the reference
in doc/api/addons.md was missed.  This fixes that
instance.

Fixes: nodejs#20645
PR-URL: nodejs#20659
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: mhdawson

doc/api/addons.md

@@ -219,7 +219,7 @@ illustration of how it can be used.
 
 ## N-API
 
-> Stability: 1 - Experimental
+> Stability: 2 - Stable
 
 N-API is an API for building native Addons. It is independent from
 the underlying JavaScript runtime (e.g. V8) and is maintained as part of