[resize-observer-1][editorial] Fix broken "viewport" reference

It's no longer defined in css-position-3, link to CSS2 instead.

Fixes w3c#8368

Author: Loirooriol

resize-observer-1/Overview.bs

@@ -24,8 +24,8 @@ urlPrefix: https://drafts.csswg.org/css-box-3/
     url: #padding-physical; type: dfn; text: padding top
 urlPrefix: https://drafts.csswg.org/css-box-3/
     url: #padding-physical; type: dfn; text: padding left
-urlPrefix: https://www.w3.org/TR/css3-positioning/
-    url: #viewport; type: dfn; text: viewport
+urlPrefix: https://www.w3.org/TR/CSS2/visuren.html; spec: CSS2;
+    url: #x1; text: viewport; type: dfn;
 urlPrefix: https://www.w3.org/TR/intersection-observer/
     url: #intersection-observer-interface; type: interface; text: IntersectionObserver
 urlPrefix: https://www.w3.org/TR/SVG2/