[Doc] Prevent unintended auto-correct with after_generate

Some `rails generators` may not generate `*.rb`. (e.g. `bin/rails g stimulus`)
In this case, `parsable_files` is empty, so RuboCop will parse all files.
This behavior is not intended, so it fixes the doc.

Author: sinsoku

README.md

@@ -72,7 +72,9 @@ module YourCoolApp
   class Application < Rails::Application
     config.generators.after_generate do |files|
       parsable_files = files.filter { |file| file.end_with?('.rb') }
-      system("bundle exec rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
+      unless parsable_files.empty?
+        system("bundle exec rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
+      end
     end
   end
 end

docs/modules/ROOT/pages/usage.adoc

@@ -42,7 +42,9 @@ module YourCoolApp
   class Application < Rails::Application
     config.generators.after_generate do |files|
       parsable_files = files.filter { |file| file.end_with?('.rb') }
-      system("bundle exec rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
+      unless parsable_files.empty?
+        system("bundle exec rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
+      end
     end
   end
 end