apps: Fixing bug with ns3::ndn::ConsumerWindow::InFlight trace source

Thanks to Junxiao Shi for catching the bug.

closes #34
diff --git a/apps/ndn-consumer-window.cc b/apps/ndn-consumer-window.cc
index a2e29b3..4243966 100644
--- a/apps/ndn-consumer-window.cc
+++ b/apps/ndn-consumer-window.cc
@@ -67,7 +67,7 @@
                      MakeTraceSourceAccessor (&ConsumerWindow::m_window))
     .AddTraceSource ("InFlight",
                      "Current number of outstanding interests",
-                     MakeTraceSourceAccessor (&ConsumerWindow::m_window))
+                     MakeTraceSourceAccessor (&ConsumerWindow::m_inFlight))
     ;
 
   return tid;