docs: Update README.md (#854)

Fixed the markdown formatting issue

Author: pengpengzx

README.md

@@ -102,7 +102,7 @@ render(<Picker />, mountNode);
 ### RangePicker
 
 | Property | Type | Default | Description |
-| --- | --- | --- | --- | --- |
+| --- | --- | --- | --- |
 | prefixCls | String | rc-picker | prefixCls of this component |
 | className | String | '' | additional css class of root dom |
 | style | React.CSSProperties |  | additional style of root dom node |