List docs use `rowIndex` in example `rowRenderer`

The [List example](https://github.com/bvaughn/react-virtualized/blob/master/docs/List.md#examples) in the docs uses the proposed-but-not-implemented `rowIndex`:

``` js
function rowRenderer ({ key, rowIndex, style}) {
  return (
    <div
      key={key}
      style={style}
    >
      {list[rowIndex]}
    </div>
  )
}
```
