fix a bug in the docs for using schema downgrading

[ssteinbach]

The documentation left out a required argument for using the downgrading system from python in one of its examples. Also clarifies how to get core schemas from python.

[codecov-commenter]

Codecov Report

Merging #1641 (85e13f8) into main (45c6cfd) will decrease coverage by 0.01%. The diff coverage is n/a.

Additional details and impacted files

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