message: docs: fix link.
diff: @@ -85,7 +85,7 @@ as its output message type. The implementation needs to set
  Multiple `google.api.HttpBody <https://github.com/googleapis/googleapis/blob/master/google/api/httpbody.proto>`_
  can be send by the gRPC server in the server streaming case.
  In this case, HTTP response header `Content-Type` will use the `content-type` from the first
  -`google.api.HttpBody <https://github.com/googleapis/googleapis/blob/master/google/api/httpbody.proto>`.
  +`google.api.HttpBody <https://github.com/googleapis/googleapis/blob/master/google/api/httpbody.proto>`_.
  Headers
  --------
  