Enhancement: Adding missing return type to pi_estimator function

[GitandTonic]

• Add -> 'None' return type annotation to pi_estimator function
• Improves code clarity and follows Python type hinting best practices
• Function already had proper type hints for parameters but added 'None' return type for better clarity as an enhancement

Describe your change:

The code works perfectly however it does not give an output in return. The addition to the return type gives more clarity and adheres Python type hinting best practices

• Enhancement in an existing algorithm

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: "Enhances monte_carlo.py".