fix: step 6 in observeProperty method

Reverse the logic "active subscription **does exist**"

fix #540

Author: relu91

index.html

@@ -2054,7 +2054,7 @@ The subscribeEvent() method
             with a {{TypeError}} and stop.
           li>
           <li>
-            If |thing|.{{ConsumedThing/[[activeSubscriptions]]}}[|eventName|] does not [=map/exist=], [=reject=] |promise| with a {{NotAllowedError}} and stop.
+            If |thing|.{{ConsumedThing/[[activeSubscriptions]]}}[|eventName|] does [=map/exist=], [=reject=] |promise| with a {{NotAllowedError}} and stop.
           li>
           <li>
             Let |subscription| be a new {{Subscription}} object with its <a>internal slotsa>