Document: `elementVoid` is incorrect.

Hey guys,

The section of [elementVoid](https://google.github.io/incremental-dom/#elementvoid) has some incorrect information.
```
Description

A combination of elementOpen, followed by elementClose. 
```
and
```
Usage

import { elementClose } from 'incremental-dom';
```

Since i'm new here so i don't know how to fix. 

