docs: use read_only=False so that example doesn't raise an exception.

[dhirschfeld]

I tried to run a query using the example connection from the README and it raised an error.

This change allows running a query using the example connection attributes:

engine = sa.create_engine(
    'duckdb:///:memory:',
    connect_args={
        'read_only': True,
        'config': {
            'memory_limit': '500mb'
        }
    }
)
with engine.connect() as conn:
    res = conn.execute(sa.text("select 1")).scalar_one()

CatalogException: Catalog Error: Cannot launch in-memory database in read-only mode!
<snip>
The above exception was the direct cause of the following exception:
<snip>
ProgrammingError: (duckdb.duckdb.CatalogException) Catalog Error: Cannot launch in-memory database in read-only mode!
(Background on this error at: https://sqlalche.me/e/20/f405)

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[Mause]

Thanks!