pointerTracker: fix ny coordinate using nx in get_real_pointer_position (#13723)

In get_real_pointer_position, real_y was calculated using nx instead
of ny, causing the cursor to land at the wrong Y position when using
the pointer-next/previous-monitor keyboard shortcuts.

Author: MaciejWdev

js/misc/pointerTracker.js

@@ -83,7 +83,7 @@ var PointerSwitcher = class {
         const monitor = Main.layoutManager.monitors[index];
 
         let real_x = (nx * monitor.width) + monitor.x;
-        let real_y = (nx * monitor.height) + monitor.y;
+        let real_y = (ny * monitor.height) + monitor.y;
 
         return [real_x, real_y];
     }