Remove custom font colour from Zoom link in call agenda


```
kyber::call_agenda(
  registry_url = "https://docs.google.com/spreadsheets/d/1Ys9KiTXXmZ_laBoCV2QWEm7AcnGSVQaXvm2xpi4XTSc/edit#gid=942365997",
  cohort_id = "2025-nmfs-champions-a",
  call_number = 1)
```

Produces `<font color="purple">Zoom:</font>` near top of  `agenda.md`. Remove custom font colour.