Fix kth lexicographic permutation

[yaadhuu]

Describe your change:

• Fix a bug or typo in an existing algorithm?

This pull request improves the clarity and robustness of
kth_lexicographic_permutation without changing the underlying algorithm.
It fixes a docstring inconsistency, corrects a minor comment typo, and
replaces assert with explicit input validation following Python best
practices. All existing doctests continue to pass.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.