[Docs] Fix VOCaug broken download link in the guideline

[Matrix53]

Motivation

VOCaug dataset download link is broken, so we need to provide another available link.

Modification

Provide the same file in OpenDataLab. See below:

BC-breaking (Optional)

It's just a small fix. Not a BC.

Use cases (Optional)

It's just a small fix. No new feature introduced.

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 MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.