docs: use tidb_client.db_engine in README example (fixes #193)

[haseebpvt]

Fixes #193

The README previously used an undefined engine variable in the
"Join Structured and Unstructured Data" example.

This change updates the code example to use tidb_client.db_engine,
making the example explicit and reducing confusion for new users.

Example after fix:

# Use the db_engine from TiDBClient when creating a Session
with Session(tidb_client.db_engine) as session:
    query = (
        select(Chunk)
        .join(User, Chunk.user_id == User.id)
        .where(User.name == "Alice")
    )
    chunks = session.exec(query).all()

[haseebpvt]

Somehow, even after adding the signing keys, my commits are still not showing the “Verified” badge. I’ll try one last time by closing this PR and reopening it from a fresh branch, since I’m not sure if the issue is related to all commits needing to be signed.