[css-grid] Improve issue wording

Author: fantasai

css-grid/Overview.bs

@@ -2874,9 +2874,8 @@ Find the Size of an ''fr''</h4>
 	</ol>
 
 	<p class='issue'>
-		This probably needs to be adjusted to handle sum(flex factors) approaches zero,
-		same as how Flexbox does it.
-
+		This section needs adjustment to handle sum(flex-factors) < 1
+		the <a href="http://www.w3.org/TR/css-flexbox-1/#resolve-flexible-lengths">same way Flexbox now does it</a>.
 
 <h2 id='pagination'>
 Fragmenting Grid Layout</h2>