Migration guide needs to update text on stubbing unit by default

[dalewking]

The migration guide says this: https://github.com/mockative/mockative/blob/main/wiki/MIGRATING-TO-2.md#implicit-stubbing-of-unit-returning-functions

That may have been true in early versions of V2 but is no longer true today. The text here needs to be updated to reflect the situation based on versions