Update classifier model file link in README

[anuragg1209]

Motivation and Context

--- Update classifier model file link in README to the new default model.

Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

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How Has This Been Tested?

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Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• A entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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[Copilot]

Pull Request Overview

Updates the README documentation to reference a new default classifier model file. The change replaces the old classifier model filename with a finetuned version that appears to be the new default model.

• Updated classifier model file link to reference the finetuned version

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[gemini-code-assist]

Code Review

This pull request updates the manual download link for the classifier model in the README.md file to point to the new default fine-tuned model. I've cross-referenced this change with src/tabpfn/model_loading.py and can confirm that tabpfn-v2-classifier-finetuned-zk73skhh.ckpt is the new default model. The change is accurate and ensures the documentation is up-to-date for users performing manual installations.