message: fix outdated example
diff: @@ -213,7 +213,8 @@ impl<Node: Hashable> CommitmentTree<Node> {
  /// sapling::Node,
  /// };
  ///
  -/// let mut rng = OsRng::new().unwrap();
  +/// let mut rng = OsRng;
  +///
  /// let mut tree = CommitmentTree::<Node>::new();
  ///
  /// tree.append(Node::new(Fr::random(&mut rng).into_repr()));
  