Update index.mdx - Known MCM issue mentioned twice

[davselliTigera]

This known issue is mentioned twice:

Multi-cluster management users only. If the manager-tls and internal-manager-tls secrets have overlapping DNS names, components such as es-calico-kube-controllers will log certificate errors. If you have previously installed a version older than v3.13.0 and never changed your manager-tls secret from the tigera-operator namespace, you must delete both of these secrets. This applies to you if the following command prints a certificate: $ kubectl get secret manager-tls -n tigera-operator -o "jsonpath={.data['cert']}".

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