Add missing close paren in Racket listing

[siadat]

Issue

One of the Racket listings is missing a closing paren.

Solution

This PR adds the missing paren.

Verification

• Checked the book.pdf file generated by make with \def\racketEd{1}
• Checked the following program

#lang racket

(struct Int (value))
(struct Prim (op args))
(struct Program (info body))
(define rd (Prim 'read '()))
(define neg-eight (Prim '- (list (Int 8))))
(define ast1_1 (Prim '+ (list rd neg-eight)))

(define (is_exp ast)
  (match ast
    [(Int n) #t]
    [(Prim 'read '()) #t]
    [(Prim '- (list e)) (is_exp e)]
    [(Prim '+ (list e1 e2))
     (and (is_exp e1) (is_exp e2))]
    [(Prim '- (list e1 e2))
     (and (is_exp e1) (is_exp e2))]
    [else #f]))

(define (is_Lint ast)
  (match ast
    [(Program '() e) (is_exp e)]
    [else #f]))

(is_Lint (Program '() ast1_1))
(is_Lint (Program '() (Prim '* (list (Prim 'read '())
                                     (Prim '+ (list (Int 8)))))))

It returns

$ racket Lint.rkt
#t
#f