I am not familiar with iOS programming but it seems your end point is invalid. As a result you will get "page not found" html response.
Instead of using http://stackexchange.com/oauth /2.2/users?order=desc&sort=reputation&site=stackoverflow&Client_Id=5029, try to use https://api.stackexchange.com/2.2/users?order=desc&sort=reputation&site=stackoverflow&Client_Id=5029 and you will get the list of users.
Hope this helps