Updating contribution docs

What does this MR do and why?

Solves #434913 (closed) by fixing a broken link.

Wrong link: https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows.html#contributing-to-the-gitlab-enterprise-edition-ee

Right link: https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributing-to-the-gitlab-enterprise-edition-ee

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