Install scikit-learn instead of sklearn

[toshihikoyanase]

Motivation

CI failed due to ModuleNotFoundError: No module named 'sklearn'.

• https://github.com/optuna/optuna-examples/actions/runs/3411459033
• https://github.com/optuna/optuna-examples/actions/runs/3411457484
• https://github.com/optuna/optuna-examples/actions/runs/3411452466

This is because sklearn package is newly registered to deprecate pip install sklearn and encourage pip install scikit-learn.

Description of the changes

• Replace sklearn in requirements.txt with scikit-learn.

[c-bata]

LGTM.