Merge branch 'docs-add-missing-column-on-changing-column-types' into 'master'

Add missing column on "Changing Column Types"

See merge request gitlab-org/gitlab!23367

doc/development/what_requires_downtime.md

@@ -213,7 +213,7 @@ class ChangeUsersUsernameStringToTextCleanup < ActiveRecord::Migration[4.2]
   disable_ddl_transaction!
 
   def up
-    cleanup_concurrent_column_type_change :users
+    cleanup_concurrent_column_type_change :users, :username
   end
 
   def down