Fix clippy error with new Rust 1.65 release (#725)

Rust 1.65 was recently released which added some new clippy rules. This
commit fixes the failure reported when running clippy with 1.65. This
would have caused a CI failure as we start using the new rust version
in CI.

src/shortest_path/num_shortest_path.rs

@@ -35,7 +35,7 @@ pub fn num_shortest_paths_unweighted<Ty: EdgeType>(
             source
         )));
     }
-    let mut bfs = Bfs::new(&graph, node_index);
+    let mut bfs = Bfs::new(graph, node_index);
     let mut distance: Vec<Option<usize>> = vec![None; graph.node_bound()];
     distance[node_index.index()] = Some(0);
     out_map[source] = 1.to_biguint().unwrap();