[css-align-3] update changes list

Author: tabatkins

css-align-3/Overview.bs

@@ -2445,6 +2445,8 @@ Changes</h2>
 		that enables [=baseline content-alignment=],
 		particularly with respect to ''margin/auto'' [=margins=].
 		(<a href="https://github.com/w3c/csswg-drafts/issues/5923">Issue 5923</a>)
+		<li>Better define how baseline self-alignment and content-alignment interact.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/6389">Issue 6389</a>)
 	</ul>
 
 	Changes since the <a href="https://www.w3.org/TR/2018/WD-css-align-3-20181206/">6 December 2018 Working Draft</a> include: