libgraphviz: use tuple indexing

Author: Jorge Aparicio

src/libgraphviz/lib.rs

@@ -218,7 +218,7 @@
 //! impl<'a> dot::Labeller<'a, Nd<'a>, Ed<'a>> for Graph {
 //!     fn graph_id(&'a self) -> dot::Id<'a> { dot::Id::new("example3").unwrap() }
 //!     fn node_id(&'a self, n: &Nd<'a>) -> dot::Id<'a> {
-//!         dot::Id::new(format!("N{}", n.val0())).unwrap()
+//!         dot::Id::new(format!("N{}", n.0)).unwrap()
 //!     }
 //!     fn node_label<'a>(&'a self, n: &Nd<'a>) -> dot::LabelText<'a> {
 //!         let &(i, _) = n;