system_tables.md: update _pico_index

Summary

• system_tables.md: update _pico_index

Address changes from picodata!1007 (merged)

Version: Picodata 24.4.0-57-g8a306f0e (picodata@8a306f0e)

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• Close #268 (closed)
• Cherry-pick to: none
• Staging: https://docs.binary.picodata.io/picodata/asabirov_update_pico_index/architecture/system_tables/#_pico_index