Remove type cast in combinations algorithm

[tianyizheng02]

Describe your change:

Replaced type cast to int in maths/combinations.py with integer division because the mathematical formula for nCr(n, k) always evaluates to an integer anyways.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• 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 have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[cclauss]

@ChrisO345 If you see PRs that have passing tests and adhere to our guidelines, and that you approve of, please go ahead an squash and merge. We should try to get the 200 open PRs down to something more reasonable.