Fix reference from "the empty list" to "an empty list"

Author: jonathantneal

css-syntax-3/Overview.bs

@@ -2529,7 +2529,7 @@ Consume a declaration</h4>
 	<a>Consume the next input token</a>.
 	Create a new declaration
 	with its name set to the value of the <a>current input token</a>
-	and its value initially set to the empty [=list=].
+	and its value initially set to an empty [=list=].
 
 	<ol>
 		<li>