POST to the correct endpoint when updating

`prefix_sms_with_service_name` is a computed attribute on the service
model. It’s where we get the value from, and the API does some work to
get it from the database, or derive it from the default SMS sender.
It can’t be updated, because it’s not itself a database column.

`prefix_sms` is the name of the actual database column. This is the
thing that we need to update.

This will go away eventually.

app/notify_client/service_api_client.py

@@ -107,7 +107,7 @@ class ServiceAPIClient(NotifyAdminAPIClient):
             'permissions',
             'organisation_type',
             'free_sms_fragment_limit',
-            'prefix_sms_with_service_name',
+            'prefix_sms',
         }
         if disallowed_attributes:
             raise TypeError('Not allowed to update service attributes: {}'.format(