Docs: fix syntax of rules array in example

What does this MR do and why?

Resolves an incorrect snippet in the "Choose when to run jobs" documentation which shows the rules keyword as an object rather than an array. Copying this snippet directly will result in a "rules should be an array of hashes" error.

How to set up and validate locally

Copy the second snippet from the "Complex Rules" section of the documentation into a GitLab pipeline. Observe that linting fails with a "rules should be an array of hashes" error.

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