Fixed bug in peekable() documentation.

### Summary

This PR fixes a bug currently present in `peekable` documentation. Current docs say:

>To test whether there are more items in the iterator, examine the peekable’s truth value. If it is truthy, there are more items.

 ```python
 assert peekable(xrange(1))
 assert not peekable([])
 ````

This does not hold true if iterable contains _falsey_ elements:

```python
items = peekable([False])
assert items.peek()
```

will raise `AssertionError` even though the `items` are not empty.