fix typo in InterpLvar example

book.tex

@@ -2163,7 +2163,7 @@ class InterpLint:
 \end{minipage}
 \begin{minipage}{0.45\textwidth}
   \begin{lstlisting}
-def InterpLvar(InterpLint):
+class InterpLvar(InterpLint):
   def interp_exp(e):
     match e:
       case Name(id):