message: Fix markdown warning with current Rust.
diff: @@ -5,7 +5,7 @@ use super::graph::IndexType;
  /// `UnionFind<K>` is a disjoint-set data structure. It tracks set membership of *n* elements
  /// indexed from *0* to *n - 1*. The scalar type is `K` which must be an unsigned integer type.
  ///
  -/// http://en.wikipedia.org/wiki/Disjoint-set_data_structure
  +/// <http://en.wikipedia.org/wiki/Disjoint-set_data_structure>
  ///
  /// Too awesome not to quote:
  ///
  