[Bug Report] Fix example message-box Promt in docs

[palpich]

Element UI version

2.4.6

OS/Browsers version

macOS

Vue version

2.5.17

Reproduction Link

https://jsfiddle.net/L9mo5d3q

Steps to reproduce

1. Input value in modal
2. Confirm

What is Expected?

Expect string value

What is actually happening?

The example message-box Promt does not work correctly, because then return object {value: 'value', action: "confirm"}