[DOC] Fix typo in MultiLabelDataset docstring

[Ezra-Yu]

Motivation

refer to #1479

The codecov has been deprecated refer to home-assistant/core#91283

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.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.79 🎉

Comparison is base (c9a0cb0) 84.37% compared to head (65839a6) 85.17%.

❗ Current head 65839a6 differs from pull request most recent head 22cf44b. Consider uploading reports for the commit 22cf44b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1483      +/-   ##
==========================================
+ Coverage   84.37%   85.17%   +0.79%     
==========================================
  Files         142      230      +88     
  Lines        9925    17492    +7567     
  Branches     1621     2742    +1121     
==========================================
+ Hits         8374    14898    +6524     
- Misses       1277     2079     +802     
- Partials      274      515     +241     

Flag	Coverage Δ
unittests	85.17% <ø> (+0.79%)	⬆️

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see 372 files with indirect coverage changes

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