docs: fix link to ThreadPool
//! [timer]: ../timer/index.html
//! [`Runtime`]: struct.Runtime.html
//! [`Reactor`]: ../reactor/struct.Reactor.html
-//! [`ThreadPool`]: ../executor/thread_pool/struct.ThreadPool.html
+//! [`ThreadPool`]: https://docs.rs/tokio-threadpool/0.1/tokio_threadpool/struct.ThreadPool.html
//! [`run`]: fn.run.html
//! [idle]: struct.Runtime.html#method.shutdown_on_idle
//! [`tokio::spawn`]: ../executor/fn.spawn.html
