HelloWorld: Cleanup

Change-Id: I19abb60e9a6db3a99955924f52c0b920b7fbd7f6
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/helloworld/helloworldwindow.h

@@ -5,16 +5,13 @@
 
 #include <QWidget>
 
-namespace HelloWorld {
-namespace Internal {
+namespace HelloWorld::Internal {
 
 class HelloWorldWindow : public QWidget
 {
     Q_OBJECT
-
 public:
     explicit HelloWorldWindow(QWidget *parent = nullptr);
 };
 
-} // namespace Internal
-} // namespace HelloWorld
+} // namespace HelloWorld::Internal