fix: helm - built-in loki-operational dashboard

[splichy]

What this PR does / why we need it:
Fix wrong job label in Chunk Flush Reason panel in helm builtin dashboard

Special notes for your reviewer:
same jo label expr is used e.g. in Chunks Flushed/Sec panel
Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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