message: docs(upgrade): fix link to upgrade example
  Closes
diff: //! See [this example][example] showing how upgrades work with both
  //! Clients and Servers.
  //!
  -//! [example]: https://github.com/hyperium/hyper/master/examples/upgrades.rs
  +//! [example]: https://github.com/hyperium/hyper/blob/master/examples/upgrades.rs
  use std::any::TypeId;
  use std::error::Error as StdError;
  