message: Switch to an example that uses Graph instead of GraphMap
diff: //! # Example
  //!
  //! ```rust
  -//! use petgraph::graphmap::UnGraphMap;
  +//! use petgraph::graph::{NodeIndex, UnGraph};
  //! use petgraph::algo::{dijkstra, min_spanning_tree};
  //! use petgraph::data::FromElements;
  //! use petgraph::dot::{Dot, Config};
  //!
  -//! // Create an undirected graph where nodes are &str, and edges have no weight.
  -//! let g = UnGraphMap::<&str, ()>::from_edges(&[
  -//! ("a", "b"), ("b", "c"), ("c", "d"),
  -//! ("a", "d")]);
  +//! // Create an undirected graph with `i32` nodes and edges with `()` associated data.
  +//! let g = UnGraph::<i32, ()>::from_edges(&[
  +//! (1, 2), (2, 3), (3, 4),
  +//! (1, 4)]);
  //!
  -//! // Find the shortest path from "a" to "d" using 1 as the cost for every edge.
  -//! let node_map = dijkstra(&g, "a", Some("d"), |_| 1);
  -//! assert_eq!(&1, node_map.get("d").unwrap());
  +//! // Find the shortest path from `1` to `4` using `1` as the cost for every edge.
  +//! let node_map = dijkstra(&g, 1.into(), Some(4.into()), |_| 1);
  +//! assert_eq!(&1i32, node_map.get(&NodeIndex::new(4)).unwrap());
  //!
  //! // Get the minimum spanning tree of the graph as a new graph, and check that
  //! // one edge was trimmed.
  -//! let mst = UnGraphMap::from_elements(min_spanning_tree(&g));
  -//! assert_eq!(g.all_edges().count() - 1, mst.all_edges().count());
  +//! let mst = UnGraph::<_, _>::from_elements(min_spanning_tree(&g));
  +//! assert_eq!(g.raw_edges().len() - 1, mst.raw_edges().len());
  //!
  -//! // Output the tree to graphviz DOT format
  +//! // Output the tree to `graphviz` `DOT` format
  //! println!("{:?}", Dot::with_config(&mst, &[Config::EdgeNoLabel]));
  //! // graph {
  -//! // 0 [label="\"a\""]
  -//! // 1 [label="\"b\""]
  -//! // 2 [label="\"c\""]
  -//! // 3 [label="\"d\""]
  -//! // 0 -- 1
  -//! // 2 -- 3
  +//! // 0 [label="\"0\""]
  +//! // 1 [label="\"0\""]
  +//! // 2 [label="\"0\""]
  +//! // 3 [label="\"0\""]
  //! // 1 -- 2
  +//! // 3 -- 4
  +//! // 2 -- 3
  //! // }
  //! ```
  //!
  