[DOCS-3989] update kubernetes command syntax

[dominik-ba]

What does this PR do?

it updates the docs regarding kubernetes exec syntax so you don't use the deprecated syntax but actually kubectl exec ... -- <command> as well as using the -c flag to define the container where you want to run this

Motivation

helping the community so they won't run into the same issues ;)

Additional Notes

nothing more :)

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[buraizu]

Thanks for the PR, created DOCS-3989 to validate internally