system_tables.md: update _pico_replicaset

Summary

• system_tables.md: update _pico_replicaset

Address changes from picodata@0a8f88d2

picodata 24.5.0-350-g0a8f88d2

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• Part of #336 (closed)
• Cherry-pick to: none
• Staging:
  • .../architecture/system_tables/#_pico_replicaset
• Follow-up for picodata!1235 (merged)