contributions.md - url이 잘못 되어서 수정

[742soft]

이슈 url 오류 수정 https://github.com/laravel/framework/discussions

[kkame]

감사합니다~

@all-contributors please add @742soft for doc

[allcontributors[bot]]

@kkame

I've put up a pull request to add @742soft! :tada: