fix docstring of Taylor Diagram add_model_set()

[jukent]

PR Summary

It says it takes floats but doesn't
Related to #176

PR Checklist

General

• Make an issue if one doesn't already exist
• Link the issue this PR resolves by adding closes #XXX to the PR description where XXX is the number of the issue.
• Add a brief summary of changes to docs/release-notes.rst in a relevant section for the next unreleased release. Possible sections include: Documentation, New Features, Bug Fixes, Internal Changes, Breaking Changes/Deprecated
• Add appropriate labels to this PR
• Make your changes in a forked repository rather than directly in this repo
• Open this PR as a draft if it is not ready for review
• Convert this PR from a draft to a full PR before requesting reviewers
• Passes precommit. To set up on your local, run pre-commit install from the top level of the repository. To manually run pre-commits, use pre-commit run --all-files and re-add any changed files before committing again and pushing.

Documentation

• Docstrings have been added to all new functions
• Docstrings have been updated with any function changes
• Internal functions have a preceding underscore (_) and have been added to docs/internal_api/index.rst
• User facing functions have been added to docs/user_api/index.rst under their module