Error in 'str' documentation

The documentation for std::str::StrSlice::slice_chars gives the following example:

``` rust
let s = "Löwe 老虎 Léopard";
assert_eq!(s.slice_chars(0, 4), "Löwe");
assert_eq!(s.slice_chars(6, 8), "老虎");
```

This does not work; the third line should be

``` rust
assert_eq!(s.slice_chars(5, 7), "老虎");
```
