Link to RDS Deployment template no longer exists

[RDS Update Cert example](https://github.com/Azure/azure-quickstart-templates/tree/master/application-workloads/rds/rds-update-certificate)

[Readme](https://github.com/Azure/azure-quickstart-templates/blob/master/application-workloads/rds/rds-update-certificate/README.md)

### Link to another quickstart template returns a 404

### Repro steps
0. Click on [RDS Deployment using existing VNET and AD](https://github.com/Azure/azure-quickstart-templates/tree/master/rds-deployment-existing-ad), which will return 404. The correct link cold be this one? https://learn.microsoft.com/en-gb/samples/azure/azure-quickstart-templates/rds-deployment-existing-ad/

Open the README for the RDS Update Cert example and inspect the link labeled "RDS Deployment using existing VNET and AD." Replace the 404 target with the correct RDS deployment template link, then verify that the link resolves and the README no longer points to the missing path.

Written by the indexing model from the issue text.