Update Zombienet documentation to clarify provider configuration

Author: nhussein11

.reference/tools/zombienet.md

@@ -110,6 +110,8 @@ provider = "INSERT_PROVIDER"
 ...
 ```
 
+Ensure to replace `INSERT_PROVIDER` with the appropriate provider: `kubernetes`, `podman`, or `native`.
+
 ### Kubernetes
 
 Kubernetes is compatible with [GKE](https://cloud.google.com/kubernetes-engine){target=\_blank}, [Docker Desktop](https://docs.docker.com/desktop/features/kubernetes/){target=\_blank}, and [kind](https://kind.sigs.k8s.io/){target=\_blank}.