[css-grid-2] Update Previous Version link

Author: fantasai

css-grid-2/Overview.bs

@@ -7,7 +7,7 @@ Group: csswg
 Work Status: exploring
 TR: https://www.w3.org/TR/css-grid-2/
 ED: https://drafts.csswg.org/css-grid-2/
-Previous version: https://www.w3.org/TR/2018/WD-css-grid-2-20180206/
+Previous version: https://www.w3.org/TR/2018/WD-css-grid-2-20180427/
 Editor: Tab Atkins Jr., Google, http://www.xanthir.com/contact/, w3cid 42199
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com, w3cid 49885