Add doctests to change_contrast

[adrian-cancio]

Describe your change:

Added doctests to change_contrast function.

Contributes to #9943

• Add or change doctests?

Checklist:

• I have read CONTRIBUTING.md
• This pull request is all my own work
• This PR only changes one algorithm file
• All functions have doctests that pass

[algorithms-keeper]

Multiple Pull Request Detected

@adrian-cancio, we are extremely excited that you want to submit multiple algorithms in this repository but we have a limit on how many pull request a user can keep open at a time. This is to make sure all maintainers and users focus on a limited number of pull requests at a time to maintain the quality of the code.

This pull request is being closed as the user already has an open pull request. Please focus on your previous pull request before opening another one. Thank you for your cooperation.

User opened pull requests (including this one): #13152, #13149, #13147, #13146