[css-writing-modes] Update "previous version" link

Author: frivoal

css-writing-modes-3/Overview.bs

@@ -8,6 +8,7 @@ Work Status: Testing
 Group: csswg
 ED: https://drafts.csswg.org/css-writing-modes-3/
 TR: https://www.w3.org/TR/css-writing-modes-3/
+Previous Version: https://www.w3.org/TR/2019/CR-css-writing-modes-3-20190903/
 Previous Version: https://www.w3.org/TR/2019/CR-css-writing-modes-3-20190730/
 Previous Version: https://www.w3.org/TR/2018/CR-css-writing-modes-3-20180524/
 Previous Version: https://www.w3.org/TR/2017/CR-css-writing-modes-3-20171207/