Specs: fix copypasta in the WheelEvent constructor

sky/specs/pointer.md

 Pointer Events
 ==============
 
 Scope
 -----
 
 The following input devices are supported by sky:
  - fingers on multitouch screens
  - mice, including mouse wheels
  - styluses on screens
@@ skipping 453 matching lines @@
                   display, in global layout coordinates
 
                y: x-position relative to the top-left corner of the
                   display, in global layout coordinates
 
 Note: The only wheels that are supported are mouse wheels and physical
 dials. Track balls are not reported as mouse wheels.
 
 ```dart
 class WheelEvent extends Event {
-  PointerEvent({ bool bubbles,
-                 this.wheel,
-                 this.delta: 0.0,
-                 this.pointer,
-                 this.x, this.y,
-               }): super(bubbles: bubbles);
+  Event({ this.wheel,
+          this.delta: 0.0,
+          this.pointer,
+          this.x, this.y,
+        }): super();
 
   final int wheel;
   final double delta; // revolutions (or fractions thereof)
   final int pointer;
   final double x; // logical pixels
   final double y; // logical pixels
 
   bool get bubbles => false;
 }
 ```