RabbitMQ: correct doc link for set_policy

[michaelklishin]

What does this PR do?

Currently the reference link is set to the guide for mirrored queues, a deprecated feature that is just one example of how policies are used

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [x] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [ ] Tests written/updated

Commits signed with GPG?

Yes

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