Update index.html

Co-authored-by: Jan Romann <[email protected]>

index.html

@@ -2054,7 +2054,7 @@ <h3>The <dfn>subscribeEvent()</dfn> method</h3>
             with a {{TypeError}} and stop.
           </li>
           <li>
-            If |thing|.{{ConsumedThing/[[activeSubscriptions]]}}[|eventName|] does [=map/exist=], [=reject=] |promise| with a {{NotAllowedError}} and stop.
+            If |thing|.{{ConsumedThing/[[activeSubscriptions]]}}[|eventName|] [=map/exists=], [=reject=] |promise| with a {{NotAllowedError}} and stop.
           </li>
           <li>
             Let |subscription| be a new {{Subscription}} object with its <a>internal slots</a>