linkify “highlight colors” in interop note

Author: delan

css-pseudo-4/Overview.bs

@@ -601,7 +601,7 @@ Styling Highlights</h3>
 -->
 
   Note: Historically (and at the time of writing)
-  only the highlight colors have been interoperably supported.
+  only the [=highlight colors=] have been interoperably supported.
 
 <h3 id=highlight-ua-styles>
 Default UA Styles</h3>