[Bug Report] popup API Error

### Mint UI version
2.2.13

### OS/Browsers version
MacOS 10.14.4 / Chrome 75.0.3770.100

### Vue version
2.5.2

### Reproduction Link
https://mint-ui.github.io/docs/#/zh-cn2/popup

### Steps to reproduce
just problem of document

### What is Expected?
popup animation normal work

### What is actually happening?
The Api option to control CSS transition of the popup, is popup-transition , not pop-transition.
关于Popup组件的动画的API 应该为 popup-transition，而不是pop-transition
The default value 'popup-fade' has not corresponding style definition, you have to implement it yourself
使用了正确的API后也没有效果，因为这段css在mint-ui中并没有体现，而是需要自己手动加上css样式。

Start with the popup documentation at the linked reproduction page and compare the documented transition option and default value with the popup component's implementation and styles. Confirm the expected animation behavior, then verify that the corrected API name and default transition work without requiring undocumented manual CSS.

Written by the indexing model from the issue text.