Use stateless functional component in dumb example

"Dumb" components like this example seem to be an obvious place to use React 0.14 stateless functional components. This PR changes the example to use one.

I don't know if a stateful component was a concious choice, but I would prefer stateless in this situation and I think it should be considered for the example.

Author: sirreal

docs/quick-start.md

@@ -42,16 +42,14 @@ It is advisable that only top-level components of your app (such as route handle
 Let’s say we have a `<Counter />` “dumb” component with a number `value` prop, and an `onIncrement` function prop that it will call when user presses an “Increment” button:
 
 ```js
-import { Component } from 'react';
-
-export default class Counter extends Component {
-  render() {
-    return (
-      <button onClick={this.props.onIncrement}>
-        {this.props.value}
-      </button>
-    )
-  }
+import React from 'react'
+
+export default function Counter(props) {
+  return (
+    <button onClick={props.onIncrement}>
+      {props.value}
+    </button>
+  )
 }
 ```