Fixed typo Num -> Int in ch 1 code.

book.tex

@@ -1205,7 +1205,7 @@ first is in \code{Lint} and the second is not.
 (Lint (Program '() ast1_1)
 (Lint (Program '()
        (Prim '- (list (Prim 'read '())
-                      (Prim '+ (list (Num 8)))))))
+                      (Prim '+ (list (Int 8)))))))
 \end{lstlisting}
 \fi}
 {\if\edition\pythonEd