manual: correct description of getListItem's return value

Hi,

just noticed that the description of `getListItem` is out of date, instead of `Just (pos, (val, widget))` it returns `Just (val, widget)`.

```haskell
getListItem :: Widget (List a b) -> Int -> IO (Maybe (ListItem a b))
type ListItem a b = (a, Widget b)
```

Best,
Markus