Editorial: Fix CreateUnmappedArgumentsObject's return-type (#2704)

CreateUnmappedArgumentsObject creates an ordinary object,
not an arguments exotic object.
(See https://tc39.es/ecma262/#sec-arguments-exotic-objects Note 1).

spec.html

@@ -14434,7 +14434,7 @@ <h1>
         <h1>
           CreateUnmappedArgumentsObject (
             _argumentsList_: unknown,
-          ): an arguments exotic object
+          ): an ordinary object
         </h1>
         <dl class="header">
         </dl>