Update email factory in README

`email.from` returns [a Hash](https://github.com/thoughtbot/griddler/blob/239fc554285c4d21fee71d9de79d88c4ebe4936e/lib/griddler/email_parser.rb#L18-L24), not a String.
Personally, I wouldn't put spaces before the paren, just like I wouldn't do `user.greeting ("hello")`