[css-display-3] Fix heading level

Author: fantasai

css-display-3/Overview.bs

@@ -1140,7 +1140,8 @@ Reordering and Accessibility</h3>
    ███    ████  ██████  ████ ████████
 -->
 
-<h3 id="visibility">Invisibility: the 'visibility' property</h3>
+<h2 id="visibility">
+Invisibility: the 'visibility' property</h2>
 
 	<pre class="propdef">
 	Name: visibility