[Feature Request] Slot and slot-scope shoud be changed in the document

### Existing Component
是

### Component Name
el-table

### Description
Hi, I found that some documents are still using slot and slot-scope. In Vue 2.6.0, it is use v-slot, your can see vue document here https://vuejs.org/v2/guide/components-slots.html.

Example:
```

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```
Now can be replaced:
```

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```
or
```

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```
Maybe there are many more places like this in the documentation, they need to change it together.