Resolve SQLAlchemy warning from RAG notebook

[andrewsykim]

The RAG notebook spits out some warning from the SQLAlchemy library:

/tmp/ipykernel_71/582989623.py:7: MovedIn20Warning: The ``declarative_base()`` function is now available as sqlalchemy.orm.declarative_base(). (deprecated since: 2.0) (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
  Base = declarative_base()