gestures: Fix silly typo on get_center() implementation

Don't mix up coordinates when calculating the bounding box center

Signed-off-by: Carlos Garnacho <carlos@lanedo.com>
Acked-by: Simon Schampijer <simon@laptop.org>
Tested-by: Manuel Kaufmann <humitos@gmail.com>

src/sugar3/event-controller/sugar-touch-controller.c

@@ -264,7 +264,7 @@ sugar_touch_controller_get_center (SugarTouchController *controller,
       x1 = MIN (x1, point->x);
       y1 = MIN (y1, point->y);
       x2 = MAX (x2, point->x);
-      y2 = MAX (y2, point->x);
+      y2 = MAX (y2, point->y);
     }
 
   if (center_x)