[css-overflow-3] Fix markup

I had accidentally made an h2 into an h3. Fix that.

Author: frivoal

css-overflow-3/Overview.bs

@@ -1050,8 +1050,8 @@ Once it is sufficiently stable in this specification,
 	</figure>
 	</div>
 
-<h3 id="block-ellipsis">
-Indicating Block-Axis Overflow: the 'block-overflow' property</h3>
+<h2 id="block-ellipsis">
+Indicating Block-Axis Overflow: the 'block-overflow' property</h2>
 
 	<pre class=propdef>
 		Name: block-overflow