doc: Added required return to example (#793)

PR-URL: nodejs/node-addon-api#793
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/function.md

@@ -30,6 +30,7 @@ Value Fn(const CallbackInfo& info) {
 
 Object Init(Env env, Object exports) {
   exports.Set(String::New(env, "fn"), Function::New<Fn>(env));
+  return exports;
 }
 
 NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)