Improve docstring style and error handling in count_vowels

[Tithi234]

What does this PR do?

• Updates the docstring to follow the repository’s preferred style
• Replaces ValueError with TypeError for invalid input types

Why is this useful?

This improves documentation consistency and aligns error handling
with standard Python practices, without changing behavior.

[Tithi234]

Thanks for taking a look!

This PR keeps all existing logic and doctests unchanged and only
improves docstring consistency and Pythonic error handling.
Happy to adjust if needed.

[algorithms-keeper]

Closing this pull request as invalid

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