Added doctests to min_cost_string_conversion.py and removed :c specifier

[Lonercode]

Describe your change:

I added doctests to the min_cost_string_conversion.py in the strings directory file as part of Hacktoberfest 2024 contributions. I also made a small change to the code by removing the ':c' specifier which made the code fail to run, and seemed necessary only if used on integers for unicode converson, not strings.

This PR contributes to #9943.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

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. To ease review, please open separate PRs for separate algorithms.
• 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.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[tianyizheng02]

Apart from one unnecessary comment, everything LGTM

[tianyizheng02]

FYI, the c specifier was already fixed by a different contributor in a previous PR: #11711

I've already merged that PR, but I'll merge your PR as well since your changes build on that PR by adding doctests. This way, both of you get to have a PR approved.