Fix Zeitwerk load error for SanitizeConfig

"expected file app/lib/sanitize_config.rb to define constant
SanitizeConfig"

app/lib/formatter.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 require 'singleton'
-require_relative './sanitize_config'
+require_relative './sanitize/config'
 
 class HTMLRenderer < Redcarpet::Render::HTML
   def block_code(code, language)
@@ -485,4 +485,4 @@ class Formatter
     "<a data-focusable=\"true\" role=\"link\" href=\"#{encode(TagManager.instance.url_for(account))}\" class=\"u-url mention\">@#{encode(account.acct)}</a>"
   end
 
-end
+end