docs: Update README.md (#873)

Author: kevinchen-pimq

README.md

@@ -45,7 +45,7 @@ render(<Picker />, mountNode);
 | className | String | '' | additional css class of root dom node |
 | style | React.CSSProperties |  | additional style of root dom node |
 | dropdownClassName | String | '' | additional className applied to dropdown |
-| dropdownAlign | Object:alignConfig of [dom-align](https://github.com/yiminghe/dom-align) |  | value will be merged into placement's dropdownAlign config |
+| popupAlign | Object:alignConfig of [dom-align](https://github.com/yiminghe/dom-align) |  | value will be merged into placement's popupAlign config |
 | popupStyle | React.CSSProperties |  | customize popup style |
 | transitionName | String | '' | css class for animation |
 | locale | Object | import from 'rc-picker/lib/locale/en_US' | rc-picker locale |