docs: update Tree's demos

components/tree/demo/basic-controlled.md

@@ -9,7 +9,7 @@
 ````jsx
 import { Tree } from 'antd';
 const TreeNode = Tree.TreeNode;
-import React, { PropTypes } from 'react';
+const PropTypes = React.PropTypes;
 
 const x = 3;
 const y = 2;

components/tree/demo/basic.md

@@ -9,7 +9,7 @@
 ````jsx
 import { Tree } from 'antd';
 const TreeNode = Tree.TreeNode;
-import React, { PropTypes } from 'react';
+const PropTypes = React.PropTypes;
 
 const Demo = React.createClass({
   propTypes: {

components/tree/demo/contextmenu.md

@@ -10,7 +10,6 @@
 import { Tree, Tooltip } from 'antd';
 import assign from 'object-assign';
 const TreeNode = Tree.TreeNode;
-import React from 'react';
 
 function contains(root, n) {
   let node = n;

components/tree/demo/draggable.md

@@ -9,7 +9,6 @@
 ````jsx
 import { Tree } from 'antd';
 const TreeNode = Tree.TreeNode;
-import React from 'react';
 
 const x = 3;
 const y = 2;

components/tree/demo/dynamic.md

@@ -9,7 +9,6 @@
 ````jsx
 import { Tree } from 'antd';
 const TreeNode = Tree.TreeNode;
-import React from 'react';
 
 function generateTreeNodes(treeNode) {
   const arr = [];