Retrieve tweeningValue from onUpdate callback in documentation (#1350)

The tweeningValue no longer seems to be available in the tween object itself. Instead, the tweeningValue is available in the tweened object that passed as a parameter to the onUpdate callback.

Author: Boydbueno

src/v2/guide/transitioning-state.md

@@ -445,8 +445,8 @@ Vue.component('animated-integer', {
 
       new TWEEN.Tween({ tweeningValue: startValue })
         .to({ tweeningValue: endValue }, 500)
-        .onUpdate(function () {
-          vm.tweeningValue = this.tweeningValue.toFixed(0)
+        .onUpdate(function (object) {
+          vm.tweeningValue = object.tweeningValue.toFixed(0)
         })
         .start()