Fix truncated tutorial code snippet.

Author: chriskohlhoff

doc/tutorial.qbk

@@ -653,6 +653,11 @@ Just as it would with a call from a single thread, concurrent calls to boost::as
   ``''''''``  boost::asio::io_context io;
   ``''''''``  printer p(io);
   ``''''''``  std::thread t([&]{ io.run(); });
+  ``''''''``  io.run();
+  ``''''''``  t.join();
+
+  ``''''''``  return 0;
+  ``''''''``}
 
 See the [link boost_asio.tutorial.tuttimer5.src full source listing]
 

example/cpp11/tutorial/timer_dox.txt

@@ -365,6 +365,7 @@ boost::asio::io_context::run() will continue to execute while there is "work" le
 do. The background thread will not exit until all asynchronous operations have
 completed.
 
+\until join
 \until }
 
 See the \ref tuttimer5src "full source listing" \n