Fixed instructions for adding Secret Variables to a CI/CD pipeline.
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
Fixes the instructions for adding Secret Variables to a CI/CD pipeline, since the UI had been reorganized. (See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9205 for the original merge request.)
Moving docs to a new location?
No. Only changed content on an existing document page.