racket: fix string interpolation in call to (error ...), figure 1.4

This corresponds to variant 3 in the documentation here:
https://docs.racket-lang.org/reference/exns.html#%28def._%28%28quote._~23~25kernel%29._error%29%29

book.tex

@@ -1450,7 +1450,7 @@ figure~\ref{fig:interp_Lint}.
     [(Prim 'read '())
      (define r (read))
      (cond [(fixnum? r) r]
-           [else (error 'interp_exp "read expected an integer" r)])]
+           [else (error 'interp_exp "read expected an integer: ~v" r)])]
     [(Prim '- (list e))
      (define v (interp_exp e))
      (fx- 0 v)]