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

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## Why are these changes needed?

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solves. -->
Fix a wrong variable name for a feature introduced in
#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

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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