Fix type for SDG acme challenge records

[btylerburton]

Follow up to these earlier changes:

• https://github.com/18F/dns/pull/653

• [ ] This is a new public-facing site (if so, please follow the additional instructions below)

  • [ ] Provide context
  • [ ] Assign to @18F/osc for review
  • [ ] TTS Digital Council's new site review process completed
  • [ ] Update the inventory with new site information