Update validation_helpers.md (#591)

Add clarification of unit

Author: benkoshy

doc/plugins/validation_helpers.md

@@ -10,7 +10,7 @@ plugin :validation_helpers
 
 Attacher.validate do
   validate_mime_type %w[image/jpeg image/png image/webp]
-  validate_max_size 5*1024*1024
+  validate_max_size 5*1024*1024 # bytes
   # ...
 end
 ```