Update NavigationService example for v3

I am trying to follow these two doc pages in conjunction:

- https://reactnavigation.org/docs/en/app-containers.html
- https://reactnavigation.org/docs/en/navigating-without-navigation-prop.html

Though the "Navigating without the navigation prop" seems out of date since the introduction of explicit app containers in v3, the **App Containers** page indicates that I can get a ref on the _container_ that functions just like a navigator. After a couple of false starts, I stumbled upon what seems to be the correct setup, but it would have been helpful to start from a working example without having to infer the correct combination.

My minimal working example:

https://snack.expo.io/@brettdh/navigationservice-and-createappcontainer

Possibly related: react-navigation/react-navigation#5252

Author: brettdh

docs/navigating-without-navigation-prop.md

@@ -11,16 +11,19 @@ You can get access to a navigator through a `ref` and pass it to the `Navigation
 ```javascript
 // App.js
 
+import { createStackNavigator, createAppContainer } from 'react-navigation';
 import NavigationService from './NavigationService';
 
 const TopLevelNavigator = createStackNavigator({ /* ... */ })
 
-class App extends React.Component {
+const AppContainer = createAppContainer(TopLevelNavigator);
+
+export default class App extends React.Component {
   // ...
 
   render() {
     return (
-      <TopLevelNavigator
+      <AppContainer
         ref={navigatorRef => {
           NavigationService.setTopLevelNavigator(navigatorRef);
         }}