Fix spago package sets' upstream url

[hjmtql]

It seems the CI fails because of the spago packeage sets' upsrteram url is outdated. So I fixed it. Please review if you are at convenience.

Error messages on CI ↓

Run spago install
spago: 
Error: Remote file not found

HTTP status code: 404

URL: https://raw.githubusercontent.com/purescript/package-sets/prepare-0.15/src/packages.dhall

Message:

1│ 404: Not Found

Error: Process completed with exit code 1.

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Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• [x] Linked any existing issues or proposals that this pull request should close
• [x] Updated or added relevant documentation in the README and/or documentation directory
• [x] Added a test for the contribution (if applicable)

[chtenb]

Thank you!