Use Title: meta instead of explicit <h1>

Author: tantek

css-display-3/Overview.bs

@@ -1,6 +1,5 @@
-<h1>CSS Display Module Level 3</h1>
-
 <pre class='metadata'>
+Title: CSS Display Module Level 3
 Status: ED
 Work Status: Refining
 ED: https://drafts.csswg.org/css-display/