docs(animations): remove duplicate word (angular#16508)

aio/content/guide/animations.md

@@ -501,7 +501,7 @@ those callbacks like this:
 
 
 
-The callbacks receive an `AnimationEvent` that contains contains useful properties such as 
+The callbacks receive an `AnimationEvent` that contains useful properties such as
 `fromState`, `toState` and `totalTime`.
 
 Those callbacks will fire whether or not an animation is picked up.