Fix environment variable parsing

[Fokko]

Reported on Slack: https://apache-iceberg.slack.com/archives/C029EE6HQ5D/p1707633685716559

export PYICEBERG_CATALOG__SOMETHING__S3__REGION=eu-north-1

Before:

>>> from pyiceberg.catalog import load_catalog
>>> load_catalog('something').properties
{'s3': {'region': 'eu-north-1'}, ...}

After:

>>> from pyiceberg.catalog import load_catalog
>>> load_catalog('something').properties
{'s3.region': 'eu-north-1', ...}

Which correspondents with the key s3.region that we use. Now the example makes sense: