[DOC] Typo in `8.2. Certificates and Secrets`

• Typo in `8.2. Certificates and Secrets`[1]
• There are `Kafka.spec.clusterCa.validityDays` twice in `8.2. Certificates and Secrets`[1]
• The latter `Kafka.spec.clusterCa.validityDays` shoud be `Kafka.spec.clientsCa.validityDays`.

[1] https://access.redhat.com/documentation/en-us/red_hat_amq/7.4/html-single/using_amq_streams_on_openshift_container_platform/index#installing-your-own-ca-certificates-str
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CA certificate validity periods, expressed as a number of days after certificate generation, can be configured in Kafka.spec.clusterCa.validityDays and Kafka.spec.clusterCa.validityDays.
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