Refactor for Ci::Config::Extendable::Entry spec

What does this MR do?

I came across this spec a little while ago and thought I could make some changes to improve it's readablity. I shortened it a little bit and used more implicit subjects to make the specs read more like technical specification.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers