Fixed a typo

README.md

@@ -35,7 +35,7 @@ or to generate the body of an email. In Rails, View generation is handled by Act
 You can read more about Action View in its [README](actionview/README.rdoc).
 
 Active Record, Active Model, Action Pack, and Action View can each be used independently outside Rails.
-In addition to them, Rails also comes with Action Mailer ([README](actionmailer/README.rdoc)), a library
+In addition to that, Rails also comes with Action Mailer ([README](actionmailer/README.rdoc)), a library
 to generate and send emails; Active Job ([README](activejob/README.md)), a
 framework for declaring jobs and making them run on a variety of queueing
 backends; Action Cable ([README](actioncable/README.md)), a framework to