Make Hanami::Action::Mime::TYPES public api

People will use these values in their app, so it's already
effectively in the public API, might as well include it in
the API docs.

lib/hanami/action/mime.rb

@@ -11,7 +11,7 @@ module Mime # rubocop:disable Metrics/ModuleLength
       # Most commom MIME Types used for responses
       #
       # @since 1.0.0
-      # @api private
+      # @api public
       TYPES = {
         txt: "text/plain",
         html: "text/html",