Update 02-writing-migration.md

[julien-leclercq]

Adds a missing import for the raw SQL migration example to work out of the box.

PR Info

• Closes no issue, should I create one?

Changes

• [x] updates the documentation so one can copy paste the code and it works

[tyt2y3]

Thank you for your suggestion. It seems like we should add ConnectionTrait to sea_orm_migration::prelude instead.

@billy1624