[css-display-3] Add issue link

Author: fantasai

css-display-3/Overview.bs

@@ -1942,7 +1942,8 @@ Changes Since 2020 Candidate Recommendation</h3>
 
 	<ul>
 		<li id="change-text-run-sequence">
-			Renamed “text run” to “[=text sequence=]”
+			Renamed “text run” to “[=text sequence=]”.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/7768">Issue 7768</a>)
 		<li id="change-root-layout">
 			Defined [=root element=] and clarified its layout as a [=block-level=]
 			in [[#root]] and the definition of [=initial containing block=].