Typo fix (#4604)

an legacy name alias → a legacy name alias

Author: MartijnCuppens

css-text-3/Overview.bs

@@ -3638,7 +3638,7 @@ Line Breaking Details</h3>
       </wpt>
     </dl>
 
-    <p>For legacy reasons, UAs must treat 'word-wrap' as an [=legacy name alias=]
+    <p>For legacy reasons, UAs must treat 'word-wrap' as a [=legacy name alias=]
       of the 'overflow-wrap' property.
 
       <wpt>