Fix typo in serializers

- thanks git merge
3 jobs for 41-alerts-endpoint in 1 minute and 4 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed whats-open-py2.7 #2991

00:01:00

passed whats-open-py3.5 #2992

00:01:04

passed whats-open-py3.6 #2993

00:01:03