chore: add optum to users list (argoproj#2466)

* add optum to users list

Signed-off-by: Travis Perdue <[email protected]>

* alphabetical

Signed-off-by: Travis Perdue <[email protected]>

Signed-off-by: Travis Perdue <[email protected]>

Author: tperdue321

USERS.md

@@ -23,6 +23,7 @@ Organizations below are **officially** using Argo Rollouts. Please send a PR wit
 1. [Nozzle](https://nozzle.io)
 1. [Opensurvey Inc.](https://opensurvey.co.kr)
 1. [OpsVerse](https://opsverse.io)
+1. [Optum](https://www.optum.com)
 1. [PagerDuty](https://www.pagerduty.com/)
 1. [PayPal](https://www.paypal.com/)
 1. [PayPay](https://paypay.ne.jp/)