fix Fig. 6.22, replace undefined n with (vector-length A)

book.tex

@@ -14280,14 +14280,13 @@ \section{Challenge: Arrays}
 \begin{figure}[tp]
 \begin{tcolorbox}[colback=white]  
 {\if\edition\racketEd
-% TODO: remove the function from the following example, like the python version -Jeremy    
 \begin{lstlisting}
 (let ([A (make-vector 2 2)])
 (let ([B (make-vector 2 3)])
 (let ([i 0])
 (let ([prod 0])
 (begin
-  (while (< i n)
+  (while (< i (vector-length A))
     (begin
       (set! prod (+ prod (* (vector-ref A i)
                             (vector-ref B i))))