message: Fix typo in explaining of shorthand types
diff: //!
  //! Each graph type vends a few shorthand type definitions that name some specific
  //! generic choices. For example, [`DiGraph<_, _>`](./graph/type.DiGraph.html) is shorthand
  -//! for [`Graph<_, _, Undirected>`](graph/struct.Graph.html).
  +//! for [`Graph<_, _, Directed>`](graph/struct.Graph.html).
  //! [`UnMatrix<_, _>`](./matrix_graph/type.UnMatrix.html) is shorthand for
  //! [`MatrixGraph<_, _, Undirected>`](./matrix_graph/struct.MatrixGraph.html). Each graph type's
  //! module documentation lists the available shorthand types.
  