docs: fix an example in docs/ci/variables/index.md

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA

What does this MR do and why?

In CI/CD YAML syntax, needs expects an array of jobs, but the example here gives a dictionary, causing an error.

This MR fixes this error.

MR acceptance checklist

N/A

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A