docs(caching): change curly brackets to square brackets

What does this MR do?

I believe there is an error in the documentation, whereby what works on my end with the latest gitlab is cache: [] instead of cache: {}.

How to setup and validate locally (strongly suggested)

Gitlab Pipeline where, you define cache key, and then they to overwrite it in some other job

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.