Update fetch.bs

Co-authored-by: Anne van Kesteren <annevk@annevk.nl>

Author: noamr

fetch.bs

@@ -8920,7 +8920,7 @@ interface FetchLaterResult {
   readonly attribute boolean activated;
 };
 
-partial interface mixin Window {
+partial interface Window {
   [NewObject] FetchLaterResult fetchLater(RequestInfo input, optional DeferredRequestInit init = {});
 };
 </pre>