Docs: fix link to io
//! [files]: fs/struct.File.html
//! [TCP]: net/struct.TcpStream.html
//! [UDP]: net/struct.UdpSocket.html
-//! [`io`]: fs/struct.File.html
+//! [`io`]: io/index.html
//! [`sync`]: sync/index.html
//! [`channel`]: channel/index.html
//!
