change doxygen index for tracing

doc/main.txt

@@ -30,8 +30,7 @@
  * The "common" module contains:
  *    - a packet class to create and manipulate simulation packets: ns3::Packet, ns3::Header, 
  *      and ns3::Trailer
- *    - a trace container class to hold lists of trace sources: ns3::TraceContainer
- *    - a Pcap file serializer which can generate pcap files from simulation packets: ns3::PcapWriter
+ *    - a set of low-level trace facilities: \ref lowleveltracing
  */
 /**
  * \namespace ns3