[css-flexbox-1] Fix issue link

Author: fantasai

css-flexbox-1/Overview.bs

@@ -3864,7 +3864,7 @@ Substantive Changes and Bugfixes</h4>
 		<li id="change-2016-baseline-alignment">
 			Change <a>flex items</a> in orthogonal flows and <a>flex items</a> without a baseline
 			to both synthesize their alignment baseline from the <a>flex item</a>’s border box.
-			(<a href="">Issue XXX</a>)
+			(<a href="https://github.com/w3c/csswg-drafts/issues/373">Issue 373</a>)
 		<li id="change-2016-main-cross-baseline">
 			Fix main/cross error in definition of <a>cross-axis baseline set</a>.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/792">Issue 792</a>)