[css-text-3] Fix grammatical error. w3c#4810

Author: fantasai

css-text-3/Overview.bs

@@ -1171,7 +1171,7 @@ Text Processing</h3>
 
     <p class="note">
       Note: The set of characters considered <a>document white space</a> (part of the document content)
-      and that considered syntactic white space (part of the CSS syntax)
+      and those considered syntactic white space (part of the CSS syntax)
       are not necessarily identical.
       However, since both include spaces (U+0020), tabs (U+0009), and line feeds (U+000A)
       most authors won't notice any differences.