[css-syntax] Fix w3c#1821.

Author: tabatkins

css-syntax/Overview.bs

@@ -1322,7 +1322,7 @@ Check if two code points are a valid escape</h4>
 	return false.
 
 	Otherwise,
-	if the second <a>code point</a> is a <a>newline</a>,
+	if the second <a>code point</a> is a <a>newline</a> or EOF,
 	return false.
 
 	Otherwise, return true.