fixes configuration examples [ci skip]

guides/source/upgrading_ruby_on_rails.md

@@ -140,7 +140,7 @@ The default configuration for Rails 6
 ```ruby
 # config/application.rb
 
-load_defaults "6.0"
+config.load_defaults "6.0"
 ```
 
 enables `zeitwerk` autoloading mode on CRuby. In that mode, autoloading, reloading, and eager loading are managed by [Zeitwerk](https://github.com/fxn/zeitwerk).
@@ -166,7 +166,7 @@ However, `classic` mode infers file names from missing constant names (`undersco
 ```ruby
 # config/application.rb
 
-load_defaults "6.0"
+config.load_defaults "6.0"
 config.autoloader = :classic
 ```
 
@@ -346,7 +346,7 @@ Applications can load Rails 6 defaults and still use the classic autoloader by s
 ```ruby
 # config/application.rb
 
-load_defaults "6.0"
+config.load_defaults "6.0"
 config.autoloader = :classic
 ```