Fix error message for WindowsPath::new

Originally discovered here: http://www.reddit.com/r/rust/comments/2dbg3j/hm_unwrap_is_being_renamed_to_assert/cjnxiax
auto merge of #16443 : steveklabnik/rust/fix_path_docs, r=kballard

Originally discovered here: http://www.reddit.com/r/rust/comments/2dbg3j/hm_unwrap_is_being_renamed_to_assert/cjnxiax