docs: fix v4 -> v5 migrations link

[Cherry]

Description

This simply updates the changelog to fix the migration info URL

Related Issue

Link to the related issue:

Type of Change (select one and follow subtasks)

• [x] Documentation (README.md)
• [ ] Maintenance or repo-level work (e.g. linting, build, tests, refactoring, etc.)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
  • [ ] I have included test coverage
• [ ] Breaking change (fix or feature that would cause existing functionality/userland code to not work as expected)
  • [ ] Explain why a breaking change is necessary:
• [ ] This change requires (or is) a documentation update
  • [ ] I have added necessary local documentation (if appropriate)
  • [ ] I have added necessary itty.dev documentation (if appropriate)

[kwhitley]

Great catch, thanks!