Use teletype for marking up the handler tracking output.

asio/src/doc/overview/handler_tracking.qbk

diff --git a/asio/src/doc/overview/handler_tracking.qbk b/asio/src/doc/overview/handler_tracking.qbk
index 3608d5a..6fd12b6 100644
--- a/asio/src/doc/overview/handler_tracking.qbk
+++ b/asio/src/doc/overview/handler_tracking.qbk
@@ -12,6 +12,7 @@
 writes debugging output to the standard error stream. The output records
 asynchronous operations and the relationships between their handlers.
 
+[teletype]
 This feature is useful when debugging and you need to know how your
 asynchronous operations are chained together, or what the pending asynchronous
 operations are. As an illustration, here is the output when you run the HTTP
@@ -96,5 +97,6 @@
 The handler tracking output may be post-processed using the included
 [^handlerviz.pl] tool to create a visual representation of the handlers
 (requires the GraphViz tool [^dot]).
+[c++]
 
 [endsect]