ini: fix spelling of the CELERY_TASK_SERIALIZER

It has "always" been wrong, and we have thus been using the default of "pickle"
- not "json" as we thought.

I doubt this change has any immediate visible impact. I guess it only means
that it will use json for results instead of pickle. That might be more stable
and debuggable.

Note: celery_config will uppercase the config settings and replace '.' with
'_', so 'celery.result.serializer' turns into 'CELERY_TASK_SERIALIZER'.

CELERY_TASK_SERIALIZER is described on
https://docs.celeryproject.org/en/3.1/configuration.html#celery-result-serializer
https://docs.celeryproject.org/en/3.1/userguide/calling.html#serializers