Change if isinstance logic to elif

Not checking if `activity` is instance of `int` when `activity` is an instance of `EmbeddedActivity` will result in better processing efficiency.

discord/channel.py

@@ -1084,7 +1084,7 @@ async def create_activity_invite(self, activity: Union[EmbeddedActivity, int] ,
         if isinstance(activity, EmbeddedActivity):
             activity = activity.value
 
-        if not isinstance(activity, int):
+        elif not isinstance(activity, int):
             raise TypeError('Invalid type provided for the activity.')
 
         return await self.create_invite(