Use Title: meta instead of explicit <h1>

Author: tantek

css-overflow-3/Overview.bs

@@ -1,5 +1,5 @@
-<h1>CSS Overflow Module Level 3</h1>
 <pre class="metadata">
+Title: CSS Overflow Module Level 3
 Status: ED
 Work Status: Revising
 ED: https://drafts.csswg.org/css-overflow/