Update migration.md

[xxxVitoxxx]

• ONLY change English documents in the Pull Request, translations will be synced and translate them with https://translate.gorm.io/ or it will cause merge conflicts!

What did this pull request do?

description of the adjustment to the changing of an existing column. Gorm does not allow changing an existing column from non-nullable to nullable to avoid encountering an error if the original column has no value. However, the existing description can be misleading.

related issue: go-gorm/gorm#4237, go-gorm/gorm#6372

[wookie0]

@xxxVitoxxx
I think your PR comment is wrong.

description of the adjustment to the changing of an existing column. Gorm does not allow changing an existing column from non-nullable to nullable to avoid encountering an error if the original column has no value. However, the existing description can be misleading.

I think the explanation below is correct.

Gorm does not allow changing an existing column from nullable to non-nullable to avoid encountering an error...

Is it right??