message: Fix broken link in documentation
diff: //! provides several [graph types](index.html#graph-types) (each differing in the
  //! tradeoffs taken in their internal representation),
  //! [algorithms](./algo/index.html#functions) on those graphs, and functionality to
  -//! [output graphs](./doc/petgraph/dot/struct.Dot.html) in
  +//! [output graphs](./dot/struct.Dot.html) in
  //! [`graphviz`](https://www.graphviz.org/) format. Both nodes and edges
  //! can have arbitrary associated data, and edges may be either directed or undirected.
  //!
  