[doc] further detail for the doc for datasets/dataset_wrappers/ClassBalancedDataset

[xiaoyuan0203]

Motivation

Further detail for the doc for datasets/dataset_wrappers/ClassBalancedDataset. The original doc seems
confusing in some ways.

Modification

I just modify the docstring for the datasets/dataset_wrappers/ClassBalancedDataset.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.

[xiaoyuan0203]

fix

[Ezra-Yu]

LGTM.