Fix typo: protoype (#834)

Author: anpol

index.bs

@@ -4156,7 +4156,7 @@ Prose accompanying an [=interface=] with a [=pair iterator=] must define a [=/li
     (a) on interfaces that do not
     [=support indexed properties=],
     or (b) with a forEach method that instead invokes its callback like
-    Set.protoype.forEach (where the key is the same as the value).
+    Set.prototype.forEach (where the key is the same as the value).
     If you’re creating an API that needs such a forEach method, please
     <a href="https://github.com/heycam/webidl/issues/new?title=Enhancement%20request%20for%20Iterables">file an issue</a>.