[css-contain-1] Use references to stable specs

The newer specs are very unstable (and marked as such), and hard to
justify as normative dependencies for TR publications for a CR-or-later
spec.

Fixes w3c#2800
Replaces w3c#2801

Author: frivoal

css-contain-1/Overview.bs

@@ -30,9 +30,9 @@ spec: css-backgrounds-3;
 </pre>
 <pre class=link-defaults>
 spec:css2; type:dfn; text:stacking context
-spec:css-content; type:property; text:content
+spec:css2; type:property; text:content
 spec:css-display-3; type:property; text:display
-spec:css-lists-3; type:property; text:counter-increment
+spec:css2; type:property; text:counter-increment
 </pre>
 <h2 id='intro'>
 Introduction</h2>