fix: authentication helm chart value example

[pbzweihander]

What's Changed in this PR

According to the greptimedb-cluster Helm chart code, name of the user in auth block should be username, not name.

https://github.com/GreptimeTeam/helm-charts/blob/1843f2a53f02c0a4ffbf65b470996bf9a2bce404/charts/greptimedb-cluster/templates/users-auth-secret.yaml#L13

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

[nicecui]

Thank you for your contribution!

[killme2008]

Thanks!