Mandelbrot: Use the new Zoom cursor

It makes it really clear now that you can use the mouse to zoom
when you see the zoom cursor.

Userland/Demos/Mandelbrot/Mandelbrot.cpp

@@ -436,6 +436,7 @@ int main(int argc, char** argv)
 
     auto app_icon = GUI::Icon::default_icon("app-mandelbrot");
     window->set_icon(app_icon.bitmap_for_size(16));
+    window->set_cursor(Gfx::StandardCursor::Zoom);
 
     return app->exec();
 }