Remove unnecessary #to_s call

The `dirs` method returns an array of strings regardless of what was
originally in the autoload paths. This type is documented:

    https://github.com/fxn/zeitwerk?tab=readme-ov-file#zeitwerkloaderdirs

Note also that the #to_s call was not tested (you could not).

railties/lib/rails/application.rb

@@ -413,7 +413,7 @@ def watchable_args # :nodoc:
       files, dirs = config.watchable_files.dup, config.watchable_dirs.dup
 
       Rails.autoloaders.main.dirs.each do |path|
-        dirs[path.to_s] = [:rb]
+        dirs[path] = [:rb]
       end
 
       [files, dirs]