Ch2 Listing 3.15: match self would be more easily understood

[ChenZhongPu]

In Listing 3.15,

   fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
     match *self {
         FileState::Open => write!(f, "OPEN"),      // <4>
         FileState::Closed => write!(f, "CLOSED"),  // <4>
     }

In fact, a & is not necessary, and using match self directly would be more easily understood.