fix typo / error regarding etcd and k8s_cluster groups (#8580)

As far as I can tell this is simply a typo that has existed from the beginning. Having it this way around (`etcd` group as a child and thus subset of `k8s_cluster`) mirrors what is written in the preceeding sentence.

docs/ansible.md

@@ -13,7 +13,7 @@ the _etcd_ group into the _k8s_cluster_, unless you are certain
 to do that and you have it fully contained in the latter:
 
 ```ShellSession
-k8s_cluster ⊂ etcd => kube_node ∩ etcd = etcd
+etcd ⊂ k8s_cluster => kube_node ∩ etcd = etcd
 ```
 
 When _kube_node_ contains _etcd_, you define your etcd cluster to be as well schedulable for Kubernetes workloads.