[minor docs fix] Fix reference to multiscale_basic_features in TrainableSegmenter

[jstriebel]

Description

This fixes the refererence in the TrainableSegmenter documentation to skimage.feature.multiscale_basic_features.

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[mkcor]

Thank you!