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