Update train.md to have runable example

[vedal]

Motivation

I wish to fix the example to make it runable
When trying it I experienced the following error:

Traceback (most recent call last):
  File "convert_balloon_to_coco.py", line 55, in <module>
    convert_balloon_to_coco(ann_file='./balloon/train/via_region_data.json',
  File "convert_balloon_to_coco.py", line 15, in convert_balloon_to_coco
    for idx, v in enumerate(track_iter_progress(data_infos.values())):
  File ".../site-packages/mmengine/utils/progressbar.py", line 240, in track_iter_progress
    raise TypeError(
TypeError: "tasks" must be a tuple object or a sequence object, but got <class 'dict_values'>

Modification

Convert dict_values to list before iterating on it

4. The documentation has been modified accordingly, like docstring or example tutorials.

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