message: docs(service): remove reference to service_fn_ok
  This function has been otherwise removed and the link was broken.
diff: //! to a single connection. It defines how to respond to **all** requests that
  //! connection will receive.
  //!
  -//! While it's possible to implement `Service` for a type manually, the helpers
  -//! [`service_fn`](service::service_fn) and
  -//! [`service_fn_ok`](service::service_fn_ok) should be sufficient for most
  -//! cases.
  +//! While it's possible to implement `Service` for a type manually, the helper
  +//! [`service_fn`](service::service_fn) should be sufficient for most cases.
  //!
  //! # MakeService
  //!
  