[two_auction] Lecture review with math and warning fixes (#556)

* remove warning and make code PEP8 compliant

* update lecture

* Update lectures/two_auctions.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: HumphreyYang