Fix issues with docstring of `gtda.mapper.method_to_transform`

[ulupo]

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description The current docstring example for the method_to_transform decorator in gtda.mapper became incorrect following changes that reshape the output of the new transform method to ensure it is at least 2D. This PR documents this reshaping in the docstring and fixes the example.

Checklist

• [x] I have read the guidelines for contributing.
• [x] My code follows the code style of this project. I used flake8 to check my Python changes.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed. I used pytest to check this on Python tests.