[css-animations] Convert to en-US spelling.

Author: dbaron

css-animations/Overview.bs

@@ -1005,7 +1005,7 @@ Types of <code>AnimationEvent</code></h3>
 
 			<p>The {{AnimationEvent/elapsedTime}} member for this event indicates
 			the number of seconds that had elapsed since the beginning of the
-			animation at the moment when the animation was cancelled.
+			animation at the moment when the animation was canceled.
 			This excludes any time where the animation was paused. If the animation
 			had a negative 'animation-delay', the beginning of the animation is the
 			moment equal to the absolute value of 'animation-delay' seconds