Fixing Instance.fromExisting (#1277)

previous with keyof CreatableInstances doesn't return proper type

include/roblox.d.ts

@@ -817,9 +817,7 @@ interface InstanceConstructor {
 	 * - This constructor will not copy any of the descendant `Instances` parented to the existing object.
 	 * - This constructor will return a new object even if the existing object had `Instance.Archivable` set to `false`.
 	 */
-	fromExisting: <T extends keyof CreatableInstances>(
-		existingInstance: CreatableInstances[T],
-	) => CreatableInstances[T];
+	fromExisting: <T extends Instance>(existingInstance: T) => T;
 }
 
 declare const Instance: InstanceConstructor;