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`.

doc/api/n-api.md

@@ -2879,15 +2879,17 @@ napi_value SayHello(napi_env env, napi_callback_info info) {
   return nullptr;
 }
 
-void Init(napi_env env, napi_value exports, napi_value module, void* priv) {
+napi_value Init(napi_env env, napi_value exports) {
   napi_status status;
 
   napi_value fn;
-  status =  napi_create_function(env, NULL, SayHello, NULL, &fn);
-  if (status != napi_ok) return;
+  status =  napi_create_function(env, nullptr, 0, SayHello, nullptr, &fn);
+  if (status != napi_ok) return nullptr;
 
   status = napi_set_named_property(env, exports, "sayHello", fn);
-  if (status != napi_ok) return;
+  if (status != napi_ok) return nullptr;
+
+  return exports;
 }
 
 NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)