Resolve "Incorrect documentation for .gitlab-ci.yml cache:key default value"

What does this MR do?

Fix the description about the default of cache:key

Are there points in the code the reviewer needs to double check?

Did we miss anything else?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
  • Has been reviewed by Documentation
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #45840 (closed)