fix: update link for Upgrade Guides in medusa/medusa README.md

[parion]

Summary

What — What changes are introduced in this PR?

I noticed the Upgrade Guide link in the @medusajs/medusa package was broken, I updated it to the proper documentation link.

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Note

Low Risk
Low risk documentation-only change; updates a single URL with no runtime or behavior impact.

Overview
Fixes a broken documentation reference in packages/medusa/README.md by updating the Upgrade Guides link to the current docs.medusajs.com/learn/update URL.

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Latest commit: 9ccc0df

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