coredns lameduck updates

[nwneisen]

These are some updates for the CoreDNS lameduck docs https://mirantis.github.io/mke-docs/docs/operations/coredns-lameduck/

• There are some commands in the doc that are not escaped correctly to be displayed as code 'kubectl edit' and 'mkectl apply'. These should use ` so they appear correctly.
• We should suggest users take a look at the configmap to verify lameduck is running. It is more specific for checking that the particular config is applied: https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/#coredns-configmap-options.