Ensure localhost is included in k8s cluster TLS cert

See explanation here:
https://github.com/rancher/k3d/issues/143\#issuecomment-552634281

--server-arg passes the --tls-san=... argument through to k3s server.
Docs for that arg are here:
https://rancher.com/docs/k3s/latest/en/installation/install-options/

examples/k8s/README.md

@@ -10,6 +10,7 @@ using wiki to drive some learning about kubernetes.
 
     mkdir -p ~/.wiki-k8s ~/workspace/fedwiki
     k3d create \
+      --server-arg --tls-san="127.0.0.1" \
       --publish 80:80 \
       -v "$HOME/.wiki-k8s:/macos/.wiki-k8s" \
       -v "$HOME/workspace/fedwiki:/macos/fedwiki" \