message: Fix type alias name
diff: @@ -151,8 +151,8 @@ public:
  using response_type
  = response_type_t<typename handle_type::signatures,
  detail::implicit_conversions_t<detail::decay_t<Ts>>...>;
  - using handle_type = response_handle<Subtype, MergePolicy<response_type>>;
  - return handle_type{dptr, std::move(ids)};
  + using result_type = response_handle<Subtype, MergePolicy<response_type>>;
  + return result_type{dptr, std::move(ids)};
  }
  /// @copydoc fan_out_request
  