doc: hello addon example should return "world"

The N-API version of the hello example, returned "hello" instead of
"world".

PR-URL: https://github.com/nodejs/node/pull/26328


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>