Fix broken "migrate to Miniflare 3" link

[ArtskydJ]

The "migrate to Miniflare 3" link in the readme points at https://miniflare.dev/get-started/migrating
That redirects to https://developers.cloudflare.com/workers/testing/miniflare/get-started/migrating, which 404s
This looks right https://developers.cloudflare.com/workers/testing/miniflare/migrations/from-v2/
So I changed the "migrate to Miniflare 3" link to point at https://miniflare.dev/migrations/from-v2

[changeset-bot]

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Latest commit: a6c5f4c

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