Add return to render

docs/optimizing-performance.md

@@ -79,11 +79,13 @@ class CounterButton extends React.Component {
   }
 
   render() {
+    return (
       <button
         color={this.props.color}
         onClick={() => this.setState(state => ({count: state.count + 1}))}>
         Count: {this.state.count}
       </button>
+    );
   }
 }
 ```
@@ -98,11 +100,13 @@ class CounterButton extends React.PureComponent {
   }
 
   render() {
+    return (
       <button
         color={this.props.color}
         onClick={() => this.setState(state => ({count: state.count + 1}))}>
         Count: {this.state.count}
       </button>
+    );
   }
 }
 ```