[Super-minor] Observer : Linked observers

[Sand3r-]

class Subject
{
Subject()
: head_(NULL)
{}

// Methods...
private:
Observer* head_;
};

I believe that the keyword "public: " has been omitted before the constructor. Obviously doesn't have much of a significance as that's not what we are here about, although it shouldn't take more than just ~30 seconds to fix.

PS. GJ on writing this marvelous book (+ making it available on web for free). I know this is not a right place to put the congratz on, but I just had to, sorry 😄