Fix error message for Path::new

Originally discovered here: http://www.reddit.com/r/rust/comments/2dbg3j/hm_unwrap_is_being_renamed_to_assert/cjnxiax
What @huonw's [example](http://play.rust-lang.org/?run=1&code=fn%20main%28%29%20%7B%0A%20%20%20%20println!%28%22%7B%7D%22%2C%20Path%3A%3Anew%28b%22abxFFcd%22%29.display%28%29%29%3B%0A%7D)? That used `Path::new()`. Change it to `WindowsPath::new()` and fix the bytestring to actually use the correct `\xFF` escape (the backslash was lost) and [it fails](http://is.gd/fE29Ll).