Fix typo in error message

[monorkin]

Hi! I ran into what I think is a typo today that confused me for a minute.

After accidentally deploying an invalid job config in recurring.yml:

  incineration:
    job: Account::IncinerateDueJob # <- invalid "job" key
    schedule: every 8 hours at minute 16

I got this error message when SolidQueue tried to boot:

Invalid recurring tasks:
- incineration: either command or class_name must be present
Exiting...

I then changed job to class_name as the error told me to, but that resulted in the same error.

  incineration:
    class_name: Account::IncinerateDueJob # <- still raises "either command or class_name must be present"
    schedule: every 8 hours at minute 16

Only after checking the documentation did I notice that the key should be class instead of class_name

  incineration:
    class: Account::IncinerateDueJob # <- this works
    schedule: every 8 hours at minute 16

This PR changes class_name to be just class in the error message and adds a test for it.

[rosa]

Great catch! Thanks a lot for fixing 🙏🏻