[css-transitions] Use spelling consistent with the DOM Events spec's preference even though both spellings are valid.

Author: dbaron

css-transitions/Overview.html

@@ -1766,7 +1766,7 @@ <h2 id=changes><span class=secno>8. </span>Changes since Working Draft of
    Draft dated 19 November 2013</a>:
 
   <ul>
-   <li>The transitionend event is no longer cancellable. This is since it has
+   <li>The transitionend event is no longer cancelable. This is since it has
     no default action, so cancelling it would have no meaning. It also
     matches the animation events.
 

css-transitions/Overview.src.html

@@ -1753,7 +1753,7 @@ <h2 id="changes">Changes since Working Draft of 19 November 2013</h2>
 dated 19 November 2013</a>:</p>
 
 <ul>
-  <li>The transitionend event is no longer cancellable.  This is since it has no default action, so cancelling it would have no meaning.  It also matches the animation events.</p>
+  <li>The transitionend event is no longer cancelable.  This is since it has no default action, so cancelling it would have no meaning.  It also matches the animation events.</p>
   <li class="issue">... ADD CHANGES HERE ...
 </ul>