[Doc] Fix link to prefix caching design

[sarckk]

Essential Elements of an Effective PR Description Checklist

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Purpose

Link to prefix caching design in https://docs.vllm.ai/en/latest/features/automatic_prefix_caching.html doesn't work. Fixes link to correct markdown.

Test Plan

CI

Test Result

CI

[github-actions]

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[gemini-code-assist]

Code Review

This pull request resolves a broken link in the Automatic Prefix Caching documentation. The updated link correctly directs to the prefix_caching.md design document. The change is accurate and improves the documentation's usability. The fix is correct as is, and I have no further recommendations.

[hmellor]

Good catch, this was linking to the old V0 version which no longer exists