system_tables.md: update _pico_replicaset

Summary

• system_tables.md: update _pico_replicaset

Address changes from picodata@7f1935b1

picodata 24.5.0-184-g7f1935b1

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• Close #308 (closed)
• Cherry-pick to: none
• Staging: https://docs.binary.picodata.io/picodata/asabirov_update_pico_replicaset/architecture/system_tables/#_pico_replicaset