Fix Pubsubhubbub::UnsubscribeService

app/services/pubsubhubbub/unsubscribe_service.rb

 # frozen_string_literal: true
 
-class Pubsubhubbub::SubscribeService < BaseService
+class Pubsubhubbub::UnsubscribeService < BaseService
   def call(account, callback)
     return ['Invalid topic URL', 422] if account.nil?