Official Demo window alerts for 'please select more than two item or less than one item.'

[z27fang]

1. What's the issue?
https://tree-select.react-component.vercel.app/?path=/story/rc-tree-select--basic
On this official demo page, if you go to 'check select' section and check 2 values, it will alert for the message given in title.

2. How to reproduce?

3. What's the expected behavior?
Not showing this alert and allowing for selecting arbitrary number of values, or give some explanation of why this is happening.

读了下代码似乎是用来展示onDropdownVisibleChange，是否需要标注一下用途以防误以为是bug呢？

tree-select/examples/basic.tsx

Lines 127 to 135 in c51317d

	onDropdownVisibleChange = visible => {
	const { value } = this.state;
	console.log(visible, value);
	if (Array.isArray(value) && value.length > 1 && value.length < 3) {
	window.alert('please select more than two item or less than one item.');
	return false;
	}
	return true;
	};

如有需要我可以提交一个pr对此功能再demo页面上做些介绍。