doc: fix example in node-api docs

Fixes: #39564

Signed-off-by: Michael Dawson <[email protected]>

PR-URL: #41264
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>
Reviewed-By: Harshitha K P <[email protected]>

Author: mhdawson

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