typo 2^64

book.tex

@@ -1412,7 +1412,7 @@ exit with a return code of \code{255}.  The interpreters in chapters
 \code{trapped-error}.
 \begin{ocamlx}
   In OCaml, overflow does not cause a trap; instead values ``wrap around''
-  to produce results modulo $2^{64}$.  The result of this program is
+  to produce results modulo $2^{63}$.  The result of this program is
   \key{-1223372036854775816}.
 \end{ocamlx}