[css-break-3] Fix subtitle markup

Author: fantasai

css-break-3/Overview.bs

@@ -1,5 +1,3 @@
-<h2 id=tagline class="no-num no-toc">Breaking the Web, one fragment at a time</h2>
-
 <pre class='metadata'>
 Title: CSS Fragmentation Module Level 3
 Status: ED
@@ -19,6 +17,8 @@ Ignored Terms: background positioning area, region chain, … message topic …,
 At Risk: the ''region'' and ''break-after/avoid-region'' values of 'break-*'
 </pre>
 
+<h1>CSS Fragmentation Module Level 3 <br> <small>Breaking the Web, one fragment at a time</h1>
+
 <h2 id="intro">
 Introduction</h2>