update tutorial documentation for SetGroupName

doc/tutorial/source/data-collection.rst

@@ -321,6 +321,7 @@ the data out of the probed Packet object:
   {
     static TypeId tid = TypeId ("ns3::Ipv6PacketProbe")
       .SetParent<Probe> ()
+      .SetGroupName ("Stats")
       .AddConstructor<Ipv6PacketProbe> ()
       .AddTraceSource ( "Output",
                         "The packet plus its IPv6 object and interface that serve as the output for this probe",

doc/tutorial/source/tracing.rst

@@ -365,6 +365,7 @@ simple Object we can work with.
     {
       static TypeId tid = TypeId ("MyObject")
         .SetParent (Object::GetTypeId ())
+        .SetGroupName ("MyGroup")
         .AddConstructor<MyObject> ()
         .AddTraceSource ("MyInteger",
                          "An integer value to trace.",