Clarify CI script needing to quote colons
## What does this MR do?
Clarifies documentation for `.gitlab-ci.yml` `script`.
## Are there points in the code the reviewer needs to double check?
Should this be a note? Is there a link to YAML spec we can point to?
## Why was this MR needed?
## What are the relevant issue numbers?
Closes#20177.
## Screenshots (if relevant)
See merge request !5481
a5fdee61b6