[doc] fix autoscaling readme

[kr11]

Pull Request Description

1. Change the target namespace in config/samples/autoscaling_v1alpha1_mock_llama.yaml from default to aibrix-system to adapt to namespace changes in docs/development/app/deployment.yaml.
2. Remove some duplicate paragraphs.

Related Issues

Resolves: #[Insert issue number(s)]

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[Jeffwan]

The change looks good to me