[css-animations-1] Update event constructor link to point to WHATWG DOM spec

Author: birtles

css-animations-1/Overview.bs

@@ -38,7 +38,7 @@ Link Defaults: css-values-3 (type) <time>, cssom-1 (interface) cssstyledeclarati
 Ignored Terms: domstring, float, animationeventinit, event, eventinit, eventtarget, document
 </pre>
 <pre class="anchors">
-url: http://w3c.github.io/dom/#constructing-events; type: dfn; text: event constructor;
+url: https://dom.spec.whatwg.org/#constructing-events; type: dfn; text: event constructor;
 urlPrefix: https://html.spec.whatwg.org/multipage/webappapis.html; type: dfn; spec: html
     text: event handlers
     text: event handler event type