doc: update example in module registration

Update return type of `Init` function in documentation to match
`napi_addon_register_func` signature. Return type used to be
`void`, now it is `napi_value`.

PR-URL: https://github.com/nodejs/node/pull/17424


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>