QThreadPool: Add missing semicolon after class in documentation

Makes it easier to copy and paste the snippet into a code editor.

Change-Id: I27c0a7aa268bd4fd0af885e929f67a28f083dabf
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/doc/snippets/code/src_corelib_concurrent_qthreadpool.cpp

@@ -55,7 +55,7 @@ class HelloWorldTask : public QRunnable
     {
         qDebug() << "Hello world from thread" << QThread::currentThread();
     }
-}
+};
 
 HelloWorldTask *hello = new HelloWorldTask();
 // QThreadPool takes ownership and deletes 'hello' automatically