message: Add a more stable link for the context hashes spec
diff: //! This module contains an implementation of Irmin's context hashes.
  //!
  -//! A document describing the algorithm can be founde [here](https://hackmd.io/DnsQrkXCRLmHA3xzLiFgGw?view).
  +//! A document describing the algorithm can be found [here](https://github.com/tarides/tezos-context-hash).
  mod ocaml;
  