Uncomment cnames for acme challenge 18F engineering and ux guides

[echappen]

reverses comments in #674

• [ ] 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