update example

Author: Terry-Su

react-16.13.1/index.js

@@ -1,5 +1,10 @@
 function App() {
-    return <div>Hello, React Source Code!</div>
+    const [count, setCount] = React.useState(0)
+    return <div>
+        <div>Hello, React Source Code!</div>
+        <div>Number: {count}</div>
+        <button onClick={() => setCount(v => v + 1)}>Increment</button>
+    </div>
 }
 
 ReactDOM.render(<App/>, document.querySelector('#app'))