Fix parens around "super"

Author: DarshalShetty

book.tex

@@ -2139,7 +2139,7 @@ \subsection{Extensible Interpreters via Method Overriding}
          (define env^ (dict-set env x v))
          ((interp_exp env^) body)]
         [else
-         (super (interp_exp env) e)]))
+         ((super interp_exp env) e)]))
     ...
   ))
 \end{lstlisting}