[css-2018] Turn off the redundant index

Part of w3c#1007

Author: frivoal

css-2018/Overview.bs

@@ -14,6 +14,7 @@ Abstract: This document collects together into one definition all the specs that
 	together form the current state of Cascading Style Sheets (CSS) as of 2018.
 	The primary audience is CSS implementers, not CSS authors, as this definition
 	includes modules by specification stability, not Web browser adoption rate.
+Boilerplate: index no
 </pre>
 <pre class=link-defaults>
 spec:css-sizing-3; type:value; for:width; text:min-content