[css-overflow-3] Markup fix.

Author: heycam

css-overflow-3/Overview.bs

@@ -505,7 +505,7 @@ as the Flow-Relative box model properties defined in [[css-logical-1#box]].
   The mapping depends on the element's 'writing-mode'.
 
 <h2 id="auto-ellipsis">
-Automatic Ellipses</h3>
+Automatic Ellipses</h2>
 
 <h3 id="text-overflow" caniuse="text-overflow">
     Overflow Ellipsis: the 'text-overflow' property</h3>