Update UPGRADING (thoughtbot#2579)

Follow thoughtbot#2481

Author: yskkin

UPGRADING

@@ -2,9 +2,9 @@
 #  NOTE FOR UPGRADING FROM 4.3.0 OR EARLIER      #
 ##################################################
 
-Paperclip is now compatible with aws-sdk >= 2.0.0.
+Paperclip is now compatible with aws-sdk-s3.
 
-If you are using S3 storage, aws-sdk >= 2.0.0 requires you to make a few small
+If you are using S3 storage, aws-sdk-s3 requires you to make a few small
 changes:
 
 * You must set the `s3_region`
@@ -13,5 +13,5 @@ changes:
   using a hyphen. For example, `:public_read` needs to be changed to
   `public-read`.
 
-For a walkthrough of upgrading from 4 to 5 and aws-sdk >= 2.0 you can watch
+For a walkthrough of upgrading from 4 to *5* (not 6) and aws-sdk >= 2.0 you can watch
 http://rubythursday.com/episodes/ruby-snack-27-upgrade-paperclip-and-aws-sdk-in-prep-for-rails-5