update docker image tag to v2.21.0 in README.md (#9802)

README.md

@@ -68,11 +68,11 @@ You will then need to use [bind mounts](https://docs.docker.com/storage/bind-mou
 to access the inventory and SSH key in the container, like this:
 
 ```ShellSession
-git checkout v2.20.0
-docker pull quay.io/kubespray/kubespray:v2.20.0
+git checkout v2.21.0
+docker pull quay.io/kubespray/kubespray:v2.21.0
 docker run --rm -it --mount type=bind,source="$(pwd)"/inventory/sample,dst=/inventory \
   --mount type=bind,source="${HOME}"/.ssh/id_rsa,dst=/root/.ssh/id_rsa \
-  quay.io/kubespray/kubespray:v2.20.0 bash
+  quay.io/kubespray/kubespray:v2.21.0 bash
 # Inside the container you may now run the kubespray playbooks:
 ansible-playbook -i /inventory/inventory.ini --private-key /root/.ssh/id_rsa cluster.yml
 ```