Fix typo in comment

So crazy story... I happened to see this pod on the cocoapods feed, and for no reason in particular I happened to look at your code and realized it looked awfully familiar in places. I then realized that the migration logic looks to have been taken from [MTMigration](https://github.com/mysterioustrousers/MTMigration), which I wrote, including a typo in this comment! I didn't notice the typo until now, so I [just fixed it](https://github.com/mysterioustrousers/MTMigration/commit/c3fc6a322cdf8fd87e6b94960b7a22913ace6b1d) in MTMigration and thought I'd make a PR to fix this one real quick too. The exception being that I think there's only one managed migration for this library, so I made it singular instead of plural.

Cool lib, and a small world!