fix(docs): cast strata to str before replace in Oregon tutorial

[okiner-3]

Summary

The Oregon tutorial's preprocessing step calls .replace() directly on the strata column (renamed from numhh_list). Since numhh_list is a pandas categorical column, mapping its values to 'signed self up + others' (a category that does not exist) raises a type error.

This adds .astype(str) before .replace() so the column is converted to a string type first, avoiding the error.

Changes

df['strata'] = df['strata'].astype(str).replace({
    'signed self up + 1 additional person': 'signed self up + others',
    'signed self up + 2 additional people': 'signed self up + others'
})