docs: Correct link for UPGRADE-20.0.md on main README (#2898)

Fixed link

Author: yutachaos

README.md

@@ -14,7 +14,7 @@ Terraform module which creates AWS EKS (Kubernetes) resources
   - [Upgrade to v17.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-17.0.md)
   - [Upgrade to v18.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-18.0.md)
   - [Upgrade to v19.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-19.0.md)
-  - [Upgrade to v20.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-19.0.md)
+  - [Upgrade to v20.x](https://github.com/terraform-aws-modules/terraform-aws-eks/blob/master/docs/UPGRADE-20.0.md)
 
 ### External Documentation