Fix the Create Fetch Event and Dispatch output to return null (#1778)

Author: sisidovski

docs/index.bs

@@ -3331,7 +3331,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
       :: |preloadResponse|, a [=promise=]
       :: |raceResponse|, a [=race response=] or null
       : Output
-      :: a [=/response=]
+      :: a [=/response=] or null
 
       1. Let |response| be null.
       1. Let |eventCanceled| be false.