Use the existing tutorial on ts changepoint to align the notebook

[BaptisteCalot]

Description

There is a tutorial on time series that we will consider as a reliable source to improve the second part of our notebook with the changepoint

Here is the tutorial : https://mapie.readthedocs.io/en/latest/examples_regression/4-tutorials/plot_ts-tutorial.html

Fixes #489

Type of change

Please remove options that are irrelevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• No unit tests

Checklist

• [ ] I have read the contributing guidelines
• [ ] I have updated the HISTORY.rst and AUTHORS.rst files
• [ ] Linting passes successfully : make lint
• [ ] Typing passes successfully : make type-check
• [ ] Unit tests pass successfully : make tests
• [ ] Coverage is 100% : make coverage
• [ ] Documentation builds successfully : make doc