Update docker executor usage documentation

doc/docker/using_docker_images.md

@@ -121,8 +121,8 @@ If you are courageous enough, you can make it fully open and accept everything:
 ```
 [runners.docker]
   image = "ruby:2.1"
-  allowed_images = ["*"]
-  allowed_services = ["*"]
+  allowed_images = ["*", "*/*"]
+  allowed_services = ["*", "*/*"]
 ```
 **It the feature is not enabled, or image isn't allowed the error message will be put into the build log.**