Add additional doctests, fix grammatical errors for maths/perfect_number.py (#12477)

* Add additional doctests for the perfect number algorithm and fix grammatical errors. Contributes to #9943

* Added newline at End of file

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

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

* Apply suggestions from code review

---------

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