Updated "how to define propTypes..."

Imported from new prop-types module.

react/README.md

@@ -557,7 +557,8 @@
   - How to define `propTypes`, `defaultProps`, `contextTypes`, etc...
 
     ```jsx
-    import React, { PropTypes } from 'react';
+    import React from 'react';
+    import PropTypes from 'prop-types';
 
     const propTypes = {
       id: PropTypes.number.isRequired,