process_ses_results: clarify log message used for unexpectedly quick callbacks

it's not quite that the notification hasn't been persisted, just
that it hasn't been given its reference (which we only receive after
sending) so can't be looked up.

Author: risicle

app/celery/process_ses_receipts_tasks.py

@@ -47,7 +47,8 @@ def process_ses_results(self, response):
                 current_app.logger.info(
                     "notification not found for reference: %(notification_reference)s "
                     "(update to %(notification_status)s). "
-                    "Callback may have arrived before notification was persisted to the DB. Adding task to retry queue",
+                    "Callback may have arrived before notification's reference was persisted to the DB. "
+                    "Adding task to retry queue",
                     extra,
                     extra=extra,
                 )