[css-ruby] Fix incorrect reference to JLReq

See https://lists.w3.org/Archives/Public/www-style/2015May/0230.html

css-ruby-1/Overview.bs

@@ -1509,7 +1509,8 @@ Overhanging Ruby: the 'ruby-overhang' property</h3>
 
 	<div class="example">
 		The user agent may use [[JIS4051]] recommendation of
-		using one ruby text character length as the maximum overhang length.
+		using <a href=https://www.w3.org/TR/css-values-4/#ic>''0.5ic''</a> (half of a full-width character) in the annotation’s font
+		as the maximum overhang length.
 		Detailed rules for how ruby text can overhang adjacent characters
 		for Japanese are described by [[JLREQ]].
 	</div>