fix typo

book.tex

@@ -1933,8 +1933,8 @@ inherits from the class for \LangInt{} and the method
 \end{minipage}
 \begin{minipage}{0.45\textwidth}
   \begin{lstlisting}
-(define interp_Lint_class
-  (class interp_Lvar_class
+(define interp_Lvar_class
+  (class interp_Lint_class
     (define/override ((interp_exp env) e)
       (match e
         [(Var x)