Update notification.yml

[danaelhe]

Trying to use an email that is not associated with the account will result in an error:

 curl -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $DIGITALOCEAN_TOKEN" \
  -d '{"name":"Landing page degraded performance","type":"latency","threshold":300,"comparison":"greater_than","notifications":{"email":["bob@gmail.com"],"slack":[{"channel":"Production Alerts","url":"https://hooks.slack.com/services/T1234567/AAAAAAAA/ZZZZZZ"}]},"period":"2m"}' \
  "https://api.digitalocean.com/v2/uptime/checks/dd70-40ed-b307-/alerts"
{"id":"bad_request","message":"invalid value for field 'notifications.email[0]': invalid email 'bob@gmail.com'","request_id":"dfd99e62bfdb"}