Readme tweak

backend/census_historical_migration/README.md

@@ -67,7 +67,7 @@ To migrate dbkeys for a given year with pagination:
 ```
 docker compose run --rm web python manage.py run_paginated_migration
   --year 2022 \
-  --batchSize 1000
+  --page_size 1000
   --pages 0, 3, 4
 ```
 - `batchSize` and `pages` are optional. The script will use default values for these if they aren't provided.