message: Add NegativeCycle example with Undirected graph
diff: @@ -556,7 +556,7 @@ pub struct NegativeCycle(());
  /// ```rust
  /// use petgraph::Graph;
  /// use petgraph::algo::bellman_ford;
  -/// use petgraph::algo::NegativeCycle;
  +/// use petgraph::prelude::*;
  ///
  /// let mut g = Graph::new();
  /// let a = g.add_node(()); // node with no weight
  @@ -590,8 +590,20 @@ pub struct NegativeCycle(());
  /// ))
  /// );
  /// // Node f (indice 5) can be reach from a with a path costing 6.
  -/// // Predecessor of f is Some(e) which predecessor is Some(d) wich predecessor is Some(a).
  +/// // Predecessor of f is Some(e) which predecessor is Some(d) which predecessor is Some(a).
  /// // Thus the path from a to f is a <-> d <-> e <-> f
  +///
  +/// let graph_with_neg_cycle = Graph::<(), f32, Undirected>::from_edges(&[
  +/// (0, 1, -2.0),
  +/// (0, 3, -4.0),
  +/// (1, 2, -1.0),
  +/// (1, 5, -25.0),
  +/// (2, 4, -5.0),
  +/// (4, 5, -25.0),
  +/// (3, 4, -1.0),
  +/// ]);
  +///
  +/// assert!(bellman_ford(&graph_with_neg_cycle, NodeIndex::new(0)).is_err());
  /// ```
  pub fn bellman_ford<G>(g: G, source: G::NodeId)
  -> Result<(Vec<G::EdgeWeight>, Vec<Option<G::NodeId>>), NegativeCycle>
  