Doc: Update Factorial Example

Updated connect syntax

Task-number: QTBUG-60635
Change-Id: Ib5280326c158221129c1817a4853bfd79bbcd895
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>

examples/widgets/statemachine/factorial/main.cpp

@@ -175,7 +175,7 @@ int main(int argc, char **argv)
 
 //! [6]
     machine.setInitialState(compute);
-    QObject::connect(&machine, SIGNAL(finished()), &app, SLOT(quit()));
+    QObject::connect(&machine, &QStateMachine::finished, &app, QCoreApplication::quit);
     machine.start();
 
     return app.exec();