Step1/04 - React Intro - final demo - fix to show the customized buttons (#131)

* Fix Step1/04 - React Intro - final

Problem:
1. Customized Button.tsx is not used in Counter.tsx
2. index.tsx still renders the 'hello world' text

Fix:
1. Use customized Button.tsx in Counter.tsx
2. Have index.tsx render the App component instead of 'hello world' text

Notes:
Validated customized button now show correctly.

* Add missing blank line.

step1-04/final/src/components/Counter.tsx

@@ -1,4 +1,5 @@
 import React from 'react';
+import { Button } from './Button';
 
 export class Counter extends React.Component<any, any> {
   constructor(props) {
@@ -13,7 +14,7 @@ export class Counter extends React.Component<any, any> {
     return (
       <div>
         {text}: {clicks}
-        <button onClick={this._onButtonClick}>Click</button>
+        <Button onClick={this._onButtonClick}>Click</Button>
       </div>
     );
   }

step1-04/final/src/index.tsx

@@ -1,4 +1,5 @@
 import React from 'react';
 import ReactDOM from 'react-dom';
+import { App } from './App';
 
-ReactDOM.render(<p>hello world </p>, document.getElementById('app'));
+ReactDOM.render(<App />, document.getElementById('app'));