Fix bad type specification in animated

Reviewed By: javache

Differential Revision: D3828502

fbshipit-source-id: 8270b00053d25f72d50f79f895ea77981e3000bb

Libraries/Animated/src/AnimatedImplementation.js

@@ -963,7 +963,7 @@ class AnimatedValueXY extends AnimatedWithChildren {
     };
   }
 
-  stopAnimation(callback?: ?() => number): void {
+  stopAnimation(callback?: (value: {x: number, y: number}) => void): void {
     this.x.stopAnimation();
     this.y.stopAnimation();
     callback && callback(this.__getValue());