examples: Set wifi data link type when PCAP file is generated

examples/tutorial/third.cc

@@ -173,8 +173,9 @@ main (int argc, char *argv[])
 
   Simulator::Stop (Seconds (10.0));
 
-  if (tracing == true)
+  if (tracing)
     {
+      phy.SetPcapDataLinkType (WifiPhyHelper::DLT_IEEE802_11_RADIO);
       pointToPoint.EnablePcapAll ("third");
       phy.EnablePcap ("third", apDevices.Get (0));
       csma.EnablePcap ("third", csmaDevices.Get (0), true);