[Data] Fix progress bars not showing % progress (ray-project#47120)

<!-- Thank you for your contribution! Please review
https://github.com/ray-project/ray/blob/master/CONTRIBUTING.rst before
opening a pull request. -->

<!-- Please add a reviewer to the assignee section when you create a PR.
If you don't have the access to it, we will shortly find a reviewer and
assign them to your PR. -->

## Why are these changes needed?

<!-- Please give a short summary of the change and the problem this
solves. -->
Fix a wrong variable name for a feature introduced in
ray-project#46699, which caused progress
bars to not show % progress / render the bar itself.

After the changes in this PR, the progress bar shows % progress as
desired:
![Screenshot at Aug 13
14-48-08](https://github.com/user-attachments/assets/f5fc5188-f33e-468c-a460-d3f115293e36)


## Related issue number

<!-- For example: "Closes ray-project#1234" -->

## Checks

- [ ] I've signed off every commit(by using the -s flag, i.e., `git
commit -s`) in this PR.
- [ ] I've run `scripts/format.sh` to lint the changes in this PR.
- [ ] I've included any doc changes needed for
https://docs.ray.io/en/master/.
- [ ] I've added any new APIs to the API Reference. For example, if I
added a
method in Tune, I've added it in `doc/source/tune/api/` under the
           corresponding `.rst` file.
- [ ] I've made sure the tests are passing. Note that there might be a
few flaky tests, see the recent failures at https://flakey-tests.ray.io/
- Testing Strategy
   - [ ] Unit tests
   - [ ] Release tests
   - [ ] This PR is not tested :(

Signed-off-by: Scott Lee <sjl@anyscale.com>

python/ray/data/_internal/execution/operators/map_operator.py

@@ -330,7 +330,7 @@ def _task_done_callback(task_index: int, exception: Optional[Exception]):
                     * self._metrics.num_outputs_of_finished_tasks
                     / self._metrics.num_tasks_finished
                 )
-                self._estimated_num_output_rows = round(
+                self._estimated_output_num_rows = round(
                     estimated_num_tasks
                     * self._metrics.rows_task_outputs_generated
                     / self._metrics.num_tasks_finished