message: Fix setup comment group_chat
diff: * based on group communication. *
  * *
  * Setup for a minimal chat between "alice" and "bob": *
  - * - ./build/bin/group_chat -s -p 4242 *
  - * - ./build/bin/group_chat -g remote:chatroom@localhost:4242 -n alice *
  - * - ./build/bin/group_chat -g remote:chatroom@localhost:4242 -n bob *
  + * - ./build/example/group_chat -s -p 4242 *
  + * - ./build/example/group_chat -g remote:chatroom@localhost:4242 -n alice *
  + * - ./build/example/group_chat -g remote:chatroom@localhost:4242 -n bob *
  \******************************************************************************/
  #include <cstdlib>
  