Update README.md

What does this MR do?

The CI_REF_SLUG variable keeps underscores (_) intact when replacing, this is mentioned differently in the docs. The better thing would be to change the behavior to match the current docs (unfortunately App Engine Apps don't allow _ but only - in the URLs for some reason).

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.