[css-transitions-2] Spell check.

In other words, convert from UK/AU to US English.

Author: dbaron

css-transitions-2/Overview.bs

@@ -367,7 +367,7 @@ are sorted in composite order (first to last) as follows:
 When determining the composite order in order to sort transition events
 where either or both of the events is a {{transitioncancel}} event,
 use the [=owning element=] and [=transition generation=]
-that were set immediately prior to cancelling the transition.
+that were set immediately prior to canceling the transition.
 
 Transitions generated using the markup defined in this specification are
 <em>not</em> added to the <a>global animation list</a> when they are created.