fix typo in 5.4

book.tex

@@ -8104,8 +8104,8 @@ uninitialized tuple on the heap during a collection.
 |$\Longrightarrow$|
   (let |$x_0$| |$e_0$| ... (let |$x_{n-1}$| |$e_{n-1}$|
   (let |$v$| (## |\itm{len}| |\itm{type}|)
-  (seq (! |$0$| |$v$| |$x_0$|) ...
-       (! |$n-1$| |$v$| |$x_{n-1}$|)
+  (seq (:= |$0$| |$v$| |$x_0$|) ...
+       (:= |$n-1$| |$v$| |$x_{n-1}$|)
        |$v$|) ... )) ...)
 \end{lstlisting}
 Actually, we can (and should) do a little better than this: any $e_i$ that is already an atom can