[css-fonts-4] Fix bikeshed syntax error

Author: litherum

css-fonts-4/Overview.bs

@@ -2464,7 +2464,7 @@ due to [[#cluster-matching]].
 
 <pre class='descdef'>
 Name: font-named-instance
-Value: auto | <string>
+Value: auto | <<string>>
 Initial: auto
 For: @font-face
 </pre>