Fix typo in serializers

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

00:01:00

passed #2992
whats-open-py3.5

00:01:04

passed #2993
whats-open-py3.6

00:01:03