docs(kubernetes): fix image name in example

[khuedoan]

Description

The current image name is incorrect:

docker pull docker.io/docker-mailserver/docker-mailserver:latest

Error response from daemon: pull access denied for docker-mailserver/docker-mailserver, repository does not exist or may require 'docker login': denied: requested access to the resource is denied

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas (none)
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works (none)
• New and existing unit tests pass locally with my changes (none)

[wernerfred]

LGTM. Thx

[github-actions]

Documentation preview for this PR is ready! 🎉

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