6.x -> 6.0 [skip ci]

This 6.x seems to be confusing to some people.

guides/source/autoloading_and_reloading_constants.md

@@ -57,7 +57,7 @@ The autoloading `zeitwerk` mode is enabled by default in Rails 6 applications ru
 
 ```ruby
 # config/application.rb
-config.load_defaults "6.x" # enables zeitwerk mode in CRuby
+config.load_defaults "6.0" # enables zeitwerk mode in CRuby
 ```
 
 In `zeitwerk` mode, Rails uses [Zeitwerk](https://github.com/fxn/zeitwerk) internally to autoload, reload, and eager load. Rails instantiates and configures a dedicated Zeitwerk instance that manages the project.