Add missing close paren in Racket listing

book.tex

@@ -1328,7 +1328,7 @@ two examples at the bottom of the figure, the first is in
     [(Program '() e) (is_exp e)]
     [else #f]))
 
-(is_Lint (Program '() ast1_1)
+(is_Lint (Program '() ast1_1))
 (is_Lint (Program '()
        (Prim '* (list (Prim 'read '())
                       (Prim '+ (list (Int 8)))))))