Fix fill_auto_in_vec function doc test

[guoard]

This PR fixes the documentation test (doctest) for the fill_auto_in_vec function, which was previously failing when running the command:

cargo test --doc

Changes Made:

• Updated the doctest to use the .into() method for converting &str to String, ensuring type consistency without cluttering the test code.
• Made the utils module public to ensure that the doctest can access the fill_auto_in_vec function correctly.

Consideration:

If these changes seem excessive or if maintaining the doctest is redundant (given that the function is already thoroughly tested in the test module within the same file), you might consider removing the doctest altogether to avoid duplicating efforts.

[coffee-cup]

thanks!