fix: make period a required field for uptime alert

[mikesmithgh]

Hi, I am working digitalocean/doctl#1419 as part of #hacktoberfest and discovered period is not defined as a required field in the spec.

If I attempt to post a request without period then I receive

{
    "id": "Internal Server Error",
    "message": "Server Error"
}

The field is also required on the UI.

[danaelhe]

GREAT catch! Will merge on green 🚀 thank you!

[danaelhe]

🚀