add import to 5.x example (react-navigation#825)

rebase of react-navigation#680

Author: morenoh149

versioned_docs/version-5.x/upgrading-from-4.x.md

@@ -80,6 +80,7 @@ The main concepts are the same. There are navigators and screens, nesting works
 - `defaultNavigationOptions` becomes `screenOptions` prop on `Navigator`
 
 ```js
+import { createStackNavigator } from '@react-navigation/stack';
 const Stack = createStackNavigator();
 
 function RootStack() {