Editorial: use sentence case for "Animation frames"

Fixes whatwg#2914.

Author: j9t

source

@@ -93468,7 +93468,7 @@ loadMySprites().then(runDemo);</pre>
 
 
 
-  <h3>Animation Frames</h3>
+  <h3>Animation frames</h3>
 
   <p>Each <code>Document</code> has a <dfn>list of animation frame callbacks</dfn>, which must be
   initially empty, and an <dfn>animation frame callback identifier</dfn>, which is a number which