docs(legacy): fix incorrect migration guide URL

[lucagobbi]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[X] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

When you click on the migration guide link in the docstring it returns a 404.

What is the new behavior?

Fix URL in docstrings in the legacy folder to properly include the /blob/main/ path segment.

Does this PR introduce a breaking change?

[ ] Yes
[X] No