Fix type of `before_script`

What does this MR do?

Fixes the documentation that says you can pass a single string, when in fact you cannot.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content