Added doctests to min_cost_string_conversion.py and removed :c specifier (#11721)

* Added doctests to min_cost_string_conversion.py and removed :c specifier

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* resolved line length issues based on ruff requirements

* modified in compliance with ruff for line length

* Update strings/min_cost_string_conversion.py

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tianyi Zheng <tianyizheng02@gmail.com>