style(Picker): adjust default value of visible-item-count to 6 (#6827)

src/datetime-picker/README.zh-CN.md

@@ -269,7 +269,7 @@ export default {
 | formatter | 选项格式化函数 | _(type, val) => val_ | - |
 | columns-order `v2.9.2` | 自定义列排序数组, 子项可选值为<br> `year`、`month`、`day`、`hour`、`minute` | _string[]_ | - |
 | item-height `v2.8.6` | 选项高度，支持 `px` `vw` `rem` 单位，默认 `px` | _number \| string_ | `44` |
-| visible-item-count | 可见的选项个数 | _number \| string_ | `5` |
+| visible-item-count | 可见的选项个数 | _number \| string_ | `6` |
 | swipe-duration `v2.2.13` | 快速滑动时惯性滚动的时长，单位`ms` | _number \| string_ | `1000` |
 
 ### DatePicker Props