[css-font-loading-3] Update outdated, broken info on promises.

Author: svgeesus

css-font-loading-3/Overview.bs

@@ -53,8 +53,8 @@ Introduction</h2>
 Values</h3>
 
 	This specification uses {{Promise}}s,
-	which are defined in <a href="http://wiki.ecmascript.org/doku.php?id=harmony:specification_drafts">ECMAScript 6</a>.
-	HTML5Rocks has some <a href="http://www.html5rocks.com/en/tutorials/es6/promises/">good tutorial material introducing Promises</a>.
+	which are defined in <a href="https://tc39.es/ecma262/multipage/control-abstraction-objects.html#sec-promise-objects">ECMAScript 6</a>.
+	MDN has some <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise">good tutorial material introducing Promises</a>.
 
 <h3 id='task-source'>
 Task Sources</h3>