Add euler project problem 15 additional solution (#12774)

* Add euler project problem 15 additional solution by explicitly counting the paths.

* Update sol2.py

* updating DIRECTORY.md

* updating DIRECTORY.md

* Trigger CI

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

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

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Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>
Co-authored-by: MaximSmolskiy <MaximSmolskiy@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>