[cssom] Fix xml-stylesheet link.

Part of w3c#6463

Author: emilio

cssom-1/Overview.bs

@@ -99,7 +99,8 @@ urlPrefix: https://url.spec.whatwg.org/#concept-
         text: URL parser
         text: URL serializer
 urlPrefix: https://www.w3.org/TR/xml-stylesheet/#dt-
-    type: dfn; text: xml-stylesheet processing instruction
+    type: dfn;
+        text: xml-stylesheet processing instruction; url: xml-stylesheet
     type: dfn; text: pseudo-attribute
 urlPrefix: https://fetch.spec.whatwg.org/#concept-
     type: dfn;