Change comment in docs example on daemon mode from referring to "D" to "-d"

The comment on line 67 is a bit confusing: it says `# D for daemon mode` when my understanding is that -d is the flag for daemon mode. It's particularly confusing for a newbie since this line uses both -d and -D, so I'm suggesting this clarification.

(I'm just getting started with Docker, so please pardon me if I'm wrong!)