Docs: margin_threshold in margin_cache.rst

[hombit]

The name of the catalog in the example was inconsistent with the default value of the margin cache threshold. I add an explicit margin_threshold argument to make the example correct

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.42%. Comparing base (fa00424) to head (a7cbeea).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #263 +/- ## ======================================= Coverage 99.42% 99.42% ======================================= Files 24 24 Lines 1221 1221 ======================================= Hits 1214 1214 Misses 7 7 ```

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