Use const reference in loop in JSON Save Game example

Fixes: QTBUG-108857
Change-Id: I503a3c9ebe145d0dae52b74435212807405f0247
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

examples/corelib/serialization/savegame/game.cpp

@@ -155,6 +155,6 @@ void Game::print(int indentation) const
     mPlayer.print(indentation + 1);
 
     QTextStream(stdout) << indent << "Levels\n";
-    for (Level level : mLevels)
+    for (const Level &level : mLevels)
         level.print(indentation + 1);
 }