update camvid download shell for train_seg doc

[DengrongGuan12]

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Motivation

update camvid download shell for train_seg doc for begginer for no confusing

Modification

update camvid download shell for train_seg doc according to openxlab official doc (https://opendatalab.com/OpenDataLab/CamVid/cli/main)

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDetection or MMPretrain.
4. The documentation has been modified accordingly, like docstring or example tutorials.