Docs: unchecked file.write() in example

In the [example for LineWriter](https://doc.rust-lang.org/std/io/struct.LineWriter.html) there is this code fragment:
```
    for &byte in road_not_taken.iter() {
       file.write(&[byte]).unwrap();
    }
```
This code isn't correct -- it should be `file.write_all` instead. In fact, trying the run the example fails on `play.rust-lang.org`.