doc: fix exporting a function example

Missing the length argument in napi_create_function.

Backport-PR-URL: #19265
PR-URL: #18661
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

doc/api/n-api.md

@@ -918,7 +918,7 @@ For example, to set a function to be returned by the `require()` for the addon:
 napi_value Init(napi_env env, napi_value exports) {
   napi_value method;
   napi_status status;
-  status = napi_create_function(env, "exports", Method, NULL, &method);
+  status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method);
   if (status != napi_ok) return NULL;
   return method;
 }