Fix enum of time_frame parameter for rate endpoint

[TimJentzsch]

Relevant issue: N/A

Description:

I forgot to add the "none" option to the time_frame parameter of the volunteer/rate endpoint, because I'm stupid. This change makes it show up properly in Swagger.

Screenshots:

Checklist:

• [X] Code Quality
• [X] Pep-8
• [ ] Tests (if applicable)
• [X] Success Criteria Met
• [ ] Inline Documentation
• [ ] Wiki Documentation (if applicable)