update wordings to match GA CLI

Author: m-chau

src/aks-preview/azext_aks_preview/custom.py

@@ -1260,7 +1260,7 @@ def aks_upgrade(cmd,
             upgrade_all = True
         else:
             msg = (
-                "Since control-plane-only argument is specified, this will upgrade only the kubernetes version of the control plane to  "
+                "Since --control-plane-only parameter is specified, this will upgrade only the kubernetes version of the control plane to  "
                 f"{instance.kubernetes_version}. Kubernetes versions of the node pools will remain unchanged, "
                 "but node image version may be upgraded if there has been cluster config change that requires VM reimage. "
                 "Continue?"