updated README to reference 'main' branch (#165)

Author: projectLewis

README.md

@@ -5,6 +5,6 @@ Platform infrastructure for the Operation Code site.
 
 ## warning
 
-This repository is using ArgoCD to deploy the Operation Code infrastructure. Changes landed on master in this repository are reflected in the real running infrastructure.
+This repository is using ArgoCD to deploy the Operation Code infrastructure. Changes landed on main in this repository are reflected in the real running infrastructure.
 
 To set up your workstation to access our Kubernetes cluster, please see [SETUP.md](SETUP.md)