use number instead of string in application.rb

guides/source/upgrading_ruby_on_rails.md

@@ -263,7 +263,7 @@ The default configuration for Rails 6
 ```ruby
 # config/application.rb
 
-config.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).