Update microk8s-setup-guide.md

[williamtrelawny]

chmod 0400 would not allow the user to edit their own ~/.kube/config file, so changed it to chmod 0600.

Summary

Short description of the change.

What changed

• List of meaningful changes

Checklist

• Tests added/updated
• Documentation updated
• This PR includes a new feature
• This PR includes a bugfix
• This PR includes a refactor

Testing Checklist

Static Validation

• helm lint ./graylog passes
• helm template graylog ./graylog --validate passes

Installation

• Fresh installation completes successfully
• All pods reach Running state
• helm test graylog -n graylog passes

Functional (if applicable)

• Web UI accessible and login works
• DataNodes visible in System > Data Nodes
• Inputs can be created and receive data

Upgrade (if applicable)

• Upgrade from previous release succeeds
• Scaling up/down works correctly
• Configuration changes apply correctly

Specific to this PR

• describe what was specifically tested

Notes for reviewers

• Verify all tests pass
• Sync up with the author before merging
• The commit history must be preserved - please use the rebase-merge or standard merge options