Resolved issue facebook#2971 (facebook#2989)

* Resolved issue facebook#2971

- changed h2 to h1 in template to adhere to accessibility guidelines.
- Added h1 styles to maintain same font size.

* Added header tag to template

Corrected templete to use semantically correct header tag for header instead of div

* Added className for App-title

- Added className="App-title" to h1 tag
- Changed CSS selector to target class instead of element

packages/react-scripts/template/src/App.css

@@ -14,6 +14,10 @@
   color: white;
 }
 
+.App-title {
+  font-size: 1.5em;
+}
+
 .App-intro {
   font-size: large;
 }

packages/react-scripts/template/src/App.js

@@ -6,10 +6,10 @@ class App extends Component {
   render() {
     return (
       <div className="App">
-        <div className="App-header">
+        <header className="App-header">
           <img src={logo} className="App-logo" alt="logo" />
-          <h2>Welcome to React</h2>
-        </div>
+          <h1 className="App-title">Welcome to React</h1>
+        </header>
         <p className="App-intro">
           To get started, edit <code>src/App.js</code> and save to reload.
         </p>