Correct links to verify domain for enterprise

What does this MR do and why?

Two of links on this Enterprise users page regarding instructions to verify domains, were configured to go to: https://docs.gitlab.com/ee/user/project/pages/custom_domains_ssl_tls_certification/index.html. This is instead of the section on the same Enterprise page to go here: https://docs.gitlab.com/ee/user/enterprise_user/#verified-domains-for-groups

This correction simplifies the advice given to customers.

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