Call {shutdown,destroy} on priority_scheduler destruction in C++11 example.

asio/src/examples/cpp11/executors/priority_scheduler.cpp

@@ -79,6 +79,12 @@ public:
     int priority_;
   };
 
+  ~priority_scheduler() noexcept
+  {
+    shutdown();
+    destroy();
+  }
+
   executor_type get_executor(int pri = 0) noexcept
   {
     return executor_type(*const_cast<priority_scheduler*>(this), pri);