Specs: fix copypasta in the WheelEvent constructor

Review URL: https://codereview.chromium.org/939043004

sky/specs/pointer.md

diff --git a/sky/specs/pointer.md b/sky/specs/pointer.md
index eaf17f2..5001f3d 100644
--- a/sky/specs/pointer.md
+++ b/sky/specs/pointer.md
@@ -471,12 +471,11 @@
 
 ```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)