[Bug Report] el-date-picker documentation mistake: @change instead of @input

### Element UI version
2.15.10

### OS/Browsers version
Chrome

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/iouUndertaken/pen/QWxBwYp

### Steps to reproduce
create a picker. use :value and @change
https://element.eleme.io/#/en-US/component/date-picker#events

### What is Expected?
having @input as event inside documentation instead @change

### What is actually happening?
it is written @change
on @change the picker doesn't do anything
on @input the picker trigger