Change sklearn to scikit-learn in requirements

[bencwallace]

Motivation

The correct package name for scikit-learn is scikit-learn, not sklearn (see the sklearn entry on PyPI).

Modification

This PR changes the sklearn requirement to scikit-learn.

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.

[YuanLiuuuuuu]

Please fix the lint

[OpenMMLab-Assistant001]

Hi @bencwallace ！

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