fix example code type signature

book.tex

@@ -16940,7 +16940,7 @@ a local variable of the enclosing function:
 \begin{minipage}{\textwidth}
 {\if\edition\racketEd
 \begin{lstlisting}
-(define (f []) : Integer
+(define (f) : ( -> Integer)
   (let ([x 0])
     (let ([g (lambda: () : Integer x)])
       (begin