Fix README dataset examples

[avinashkamat48]

Updates two README snippets so they match the current public API:

• validate_dataset is shown with a Dataset model instead of a plain dict, matching its Dataset | str signature.
• The archive example uses the supported Zenodo dataset source already shown earlier in the README instead of a Frictionless datapackage.json URL that currently raises Invalid Dataset.

Addresses #15 and #16.

Verification:

• git diff --check

I could not execute the snippets locally because the clone does not have runtime dependencies installed (polars is missing).

[roll]

thanks! fixed in main already