[css-values-3] Update changes section

Author: tabatkins

css-values-3/Overview.bs

@@ -2223,14 +2223,16 @@ Changes</h2>
 	Changes since the <a href="https://www.w3.org/TR/2019/CR-css-values-3-20190606/">6 June 2019 Candidate Recommendation</a> are:
 
 	* Dropped the ''attr()'' function, since it was punted to Level 5.
+	* Specified serialization of empty urls to be <code>url("")</code>.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/6447">Issue 6447</a>)
 
 	Changes since the <a href="https://www.w3.org/TR/2019/CR-css-values-3-20190131/">31 January 2019 Candidate Recommendation</a> are:
 
 	<ul>
 		<li>Added the new <a>bracketed range notation</a> to the CSS <a>value definition syntax</a>.
 			This has no normative implications on implementations,
 			just allows more routine annotation of ranges in future CSS specifications.
-			<a href="https://github.com/w3c/csswg-drafts/issues/355">Issue 355</a>)
+			(<a href="https://github.com/w3c/csswg-drafts/issues/355">Issue 355</a>)
 	</ul>
 
 	Changes since the <a href="https://www.w3.org/TR/2018/CR-css-values-3-20180814/">14 August 2018 Candidate Recommendation</a> are: