Adds Doc test in depth_first_search_2.py (#10094)

* Adds Doc test in depth_first_search_2.py

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

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

* Fixes depth_first_search_2.py formatting

* Cleanup

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