Hello World: fix detach behavior

The hello world example relies on being able to exit the container attachment using Control-C without stopping it. According to this thread (and my own experience), this doesn't work anymore:

https://groups.google.com/forum/#!msg/docker-user/nWXAnyLP9-M/kbv-FZpF4rUJ

Added -sig-proxy=false to the attach command.
Add the customary bold
Merge pull request #3503 from gertvv/patch-1

Hello World: fix detach behavior