Merge pull request rails#23937 from y-yagi/use_app_update

use `app:update` instead of deprecated `rails:update` [ci skip]

Author: arunagw

guides/source/upgrading_ruby_on_rails.md

@@ -44,13 +44,13 @@ TIP: Ruby 1.8.7 p248 and p249 have marshaling bugs that crash Rails. Ruby Enterp
 
 ### The Task
 
-Rails provides the `rails:update` rake task. After updating the Rails version
+Rails provides the `app:update` rake task. After updating the Rails version
 in the Gemfile, run this rake task.
 This will help you with the creation of new files and changes of old files in an
 interactive session.
 
 ```bash
-$ rails rails:update
+$ rails app:update
    identical  config/boot.rb
        exist  config
     conflict  config/routes.rb