Fix documentation indentation:

- This was breaking the code highlight in the doc

guides/source/upgrading_ruby_on_rails.md

@@ -849,21 +849,21 @@ The configuration is by default global for all your models, but you can
 override it on a per model basis. This should help you migrate all your models to have their
 associations required by default.
 
-    ```ruby
-    class Book < ApplicationRecord
-      # model is not yet ready to have its association required by default
+```ruby
+class Book < ApplicationRecord
+  # model is not yet ready to have its association required by default
 
-      self.belongs_to_required_by_default = false
-      belongs_to(:author)
-    end
+  self.belongs_to_required_by_default = false
+  belongs_to(:author)
+end
 
-    class Car < ApplicationRecord
-      # model is ready to have its association required by default
+class Car < ApplicationRecord
+  # model is ready to have its association required by default
 
-      self.belongs_to_required_by_default = true
-      belongs_to(:pilot)
-    end
-    ```
+  self.belongs_to_required_by_default = true
+  belongs_to(:pilot)
+end
+```
 
 #### Per-form CSRF Tokens