Fix up typoes and language in README and messages.json

Hi @jalkoby, I tried Squasher on my project and it seems great. I'm surprised people don't talk about squashing migrations more often in the Rails community, it's pretty useful. Having a simple gem that does all the work makes it even better.

This is just a PR to fix up some of the language in the readme and in the user-facing strings. Hope these changes are cool with you.