[css-highlight-api] add previous version

Author: frivoal

css-highlight-api-1/Overview.bs

@@ -34,6 +34,7 @@ Group: csswg
 ED: https://drafts.csswg.org/css-highlight-api-1/
 TR: https://www.w3.org/TR/css-highlight-api-1/
 Previous Version: https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201022/
+Previous Version: https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201208/
 Editor: Florian Rivoal, On behalf of Bloomberg, https://florian.rivoal.net/, w3cid 43241
 Editor: Sanket Joshi, Microsoft Corporation https://www.microsoft.com, https://github.com/sanketj
 Editor: Megan Gardner, Apple Inc. https://apple.com/