Fix issues in the documentation of CGAL::Straight_skeleton_2<K>

Straight_skeleton_2/doc/Straight_skeleton_2/CGAL/Straight_skeleton_2.h

@@ -15,16 +15,20 @@ straight skeleton. If an advanced user needs to get access to the
 modifying operations, it must call the required methods through the
 `Base` class.
 
-\tparam Traits_ must be a model of `Kernel`
+\tparam Traits must be a model of `Kernel`
 
 \sa `StraightSkeletonVertex_2`
 \sa `StraightSkeletonHalfedge_2`
 */
-template< typename Traits_>
+template< typename Traits>
 class Straight_skeleton_2
-  : public HalfedgeDS_vector<Traits,Items,Alloc> {
+  : public CGAL::HalfedgeDS_default<Traits>
+{
 public:
 
+  /// Access to the base (HDS) class
+  typedef unspecified_type Base;
+
 }; /* end Straight_skeleton_2 */
 
 } /* end namespace CGAL */