Align class event handler creation with React docs

This is small change to align example React app with current advised way of assigning
event handlers that are declared as ES6 class methods:
https://reactjs.org/docs/handling-events.html

Thanks!

src/Microsoft.DotNet.Web.Spa.ProjectTemplates/content/React-CSharp/ClientApp/src/components/Counter.js

@@ -6,6 +6,7 @@ export class Counter extends Component {
   constructor() {
     super();
     this.state = { currentCount: 0 };
+    this.incrementCounter = this.incrementCounter.bind(this);
   }
 
   incrementCounter() {
@@ -23,7 +24,7 @@ export class Counter extends Component {
 
         <p>Current count: <strong>{this.state.currentCount}</strong></p>
 
-        <button onClick={() => this.incrementCounter()}>Increment</button>
+        <button onClick={this.incrementCounter}>Increment</button>
       </div>
     );
   }