Fix invalid link to GitLab.com architecture.md
https://about.gitlab.com/handbook/infrastructure/production-architecture/ is 404 now. Looks like https://about.gitlab.com/handbook/engineering/infrastructure/production-architecture/ is a valid URL now.
https://about.gitlab.com/handbook/infrastructure/production-architecture/ is 404 now. Looks like https://about.gitlab.com/handbook/engineering/infrastructure/production-architecture/ is a valid URL now.