Fix Imgproxy.url_for docs

Author: DarthSim

lib/imgproxy.rb

@@ -100,7 +100,7 @@ def configure
     # @option options [Boolean] :auto_rotate
     # @option options [String] :filename
     # @option options [String] :format
-    # @option options [Boolean] :attachment
+    # @option options [Boolean] :return_attachment
     # @option options [Boolean] :use_short_options
     # @option options [Boolean] :base64_encode_urls
     # @option options [Boolean] :escape_plain_url