fix: refer to PostgresContainer properties that exist for username, password, and dbname#4

alexanderankin wants to merge 1 commit intotestcontainers:maintestcontainers/tc-guide-getting-started-with-testcontainers-for-python:mainfrom

alexanderankin:fix-for-test-refer-to-existing-postgres-propertiesalexanderankin/tc-python-guide:fix-for-test-refer-to-existing-postgres-propertiesCopy head branch name to clipboard