[css-sizing-3] Shortname update.

Author: fantasai

css-sizing-3/Overview.bs

@@ -5,8 +5,8 @@ Level: 3
 Status: WD
 Work Status: Revising
 Group: csswg
-ED: https://drafts.csswg.org/css-sizing
-TR: https://www.w3.org/TR/css3-sizing/
+ED: https://drafts.csswg.org/css-sizing-3/
+TR: https://www.w3.org/TR/css-sizing-3/
 Editor: Tab Atkins, Google, http://xanthir.com/contact/
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Previous version: https://www.w3.org/TR/2016/WD-css-sizing-3-20160512/