update previous version link

Author: svgeesus

css-fonts-4/Overview.bs

@@ -7,7 +7,7 @@ Work Status: Exploring
 Group: CSSWG
 ED: https://drafts.csswg.org/css-fonts-4/
 TR: https://www.w3.org/TR/css-fonts-4/
-Previous Version: https://www.w3.org/TR/2017/WD-css-fonts-4-20170711/
+Previous Version: https://www.w3.org/TR/2018/WD-css-fonts-4-20180920/
 Editor: John Daggett, Invited Expert, https://twitter.com/nattokirai, w3cid 41498
 Editor: Myles C. Maxfield, Apple Inc., mmaxfield@apple.com, w3cid 77180
 Editor: Chris Lilley, W3C, http://svgees.us, w3cid 1438