[css-contain-2] Cross-reference fix

Author: frivoal

css-contain-2/Overview.bs

@@ -43,6 +43,7 @@ spec:css-break-3; type:dfn; text:fragmentation container
 spec:css-break-3; type:dfn; text:fragmentation context
 spec:css-break-3; type:dfn; text:fragmented flow
 spec:css-sizing-4; type:property; text:contain-intrinsic-size
+spec:intersection-observer; type:dfn; text:intersection root
 </pre>
 
 <h2 id='intro'>