Fixed wheelDelta type on Typescript definition file

Changed type *WheelEventProxy* to *number* for property *wheelDelta* in *phaser.d.ts* as specified in the Phaser Documentation: http://docs.phaser.io/Phaser.Mouse.html#wheelDelta

Author: Daniel Belohlavek

typescript/phaser.d.ts

@@ -2136,7 +2136,7 @@ declare module Phaser {
         _wheelEvent: WheelEventProxy;
         pointerLock: Phaser.Signal;
         stopOnGameOut: boolean;
-        wheelDelta: WheelEventProxy;
+        wheelDelta: number;
 
         onMouseDown(event: MouseEvent): void;
         onMouseMove(event: MouseEvent): void;