Upgrade Guide: Add missing period, ✂ whitespace [skip ci]

guides/source/upgrading_ruby_on_rails.md

@@ -61,7 +61,7 @@ all models inherit from `ApplicationRecord`.
 `ApplicationRecord` is a new superclass for all app models, analogous to app
 controllers subclassing `ApplicationController` instead of
 `ActionController::Base`. This gives apps a single spot to configure app-wide
-model behavior
+model behavior.
 
 When upgrading from Rails 4.2 to Rails 5.0 you need to create an
 `application_record.rb` file in `app/models/` and add the following content: