Fix restart link on doc/pages/administration.md

The links for restart_gitlab.md and backup_restore.md both had an extra ../, resulting in a 404 on https://docs.gitlab.com/ee/pages/administration.html#nginx-configuration

As mentioned in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/967#note_20134955

mentioned in merge request !967 (merged)

merged

mentioned in commit 559dbe16

added pages label