Fix type hint for Table.update transform callback

[jayasree-007]

Summary

Update the type hint for the Table.update transform callback from
Callable[[Mapping], None] to Callable[[MutableMapping], None].

The callback is expected to mutate the document in place. Existing usage,
including the delete transform, performs operations such as del on the
document, which requires a MutableMapping rather than a Mapping.

This change aligns the type annotation with the actual behavior.

Testing

• Ran the full test suite.
• All tests passed.