MessageBox: fix type definition (#14278)

`MessageBoxData` may be a action string
https://github.com/ElemeFE/element/blob/b2f0958089a197907afdbea450a9faec69b48f02/packages/message-box/src/main.js#L57-L62

types/message-box.d.ts

@@ -2,7 +2,7 @@ import Vue, { VNode } from 'vue'
 import { MessageType } from './message'
 
 export type MessageBoxCloseAction = 'confirm' | 'cancel' | 'close'
-export type MessageBoxData = MessageBoxInputData
+export type MessageBoxData = MessageBoxInputData | MessageBoxCloseAction
 
 export interface MessageBoxInputData {
   value: string,