doc: fix example in node-api docs

Fixes: nodejs#39564

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: nodejs#41264
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>

doc/api/n-api.md

@@ -4542,6 +4542,7 @@ snippet:
 function AddTwo(num) {
   return num + 2;
 }
+global.AddTwo = AddTwo;
 ```
 
 Then, the above function can be invoked from a native add-on using the