Fixes an invalid documentation for the .gitlab-ci.yml cache:key

What does this MR do?

Changes the .gitlab-ci.yml-documentation to contain example values that work with nested branch names like feature/add-some-thing

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

Current version is misleading - see gitlab-runner#3085 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

gitlab-runner#3085 (closed)