[css-text-4] Only accept "none" as a standalone keyword for text-fit

Follow-up to #13616.

Author: cdoublev

css-text-4/Overview.bs

@@ -9746,7 +9746,7 @@ Text fitting: the 'text-fit' property</h3>
 
 	<pre class="propdef">
 	Name: text-fit
-	Value: [ none | grow | shrink ] [consistent | per-line | per-line-all]? <<percentage>>?
+	Value: none | [ grow | shrink ] [ consistent | per-line | per-line-all ]? <<percentage>>?
 	Initial: none
 	Applies to: <a>block containers</a>
 	Inherited: no