Update kubernetes.md, fix typo in `<CERTIFICATE_FILENAME>`

What does this MR do?

• Fix typo in <CERTIFICATE_FILENAME>

Why was this MR needed?

• Cosmetic variable update

Are there points in the code the reviewer needs to double check?

• Low risk spelling update

Does this MR meet the acceptance criteria?

• Documentation created/updated
• [N/A] Added tests for this feature/bug
• [N/A] In case of conflicts with master - branch was rebased

What are the relevant issue numbers?