virtual destructors and missing @end in tutorial

src/helper/csma-helper.h

@@ -50,6 +50,7 @@ public:
    * Construct a CsmaHelper.
    */
   CsmaHelper ();
+  virtual ~CsmaHelper () {}
 
   /**
    * \param type the type of queue

src/helper/point-to-point-helper.h

@@ -50,6 +50,7 @@ public:
    * point networks.
    */
   PointToPointHelper ();
+  virtual ~PointToPointHelper () {}
 
   /**
    * Each point to point net device must have a queue to pass packets through.