Update README.md

Author: aksonov

README.md

@@ -21,11 +21,11 @@ Define all your routes in one React component...
 ```js
 const App = () => (
   <Router>
-    <Scene key="root">
+    <Stack key="root">
       <Scene key="login" component={Login} title="Login"/>
       <Scene key="register" component={Register} title="Register"/>
       <Scene key="home" component={Home}/>
-    </Scene>
+    </Stack>
   </Router>
 );
 ```