Update kubernetes selfhosting documentation

[ukashazia]

Update the container arguments for the self hostk8s document.

[greptile-apps]

Greptile Summary

Removes the server arg from the Kubernetes deployment container args, so the args now correctly read ["start"] rather than ["server", "start"]. The Docker image entrypoint is atuin-server (not atuin), so the correct subcommand is start alone — consistent with docs/docs/self-hosting/docker.md (command: start) and the Dockerfile.

_{Last reviewed commit: af588f7}