fixed vector grammar to use arg

book.tex

@@ -4058,7 +4058,7 @@ forms for vectors. Here is the definition of $C_2$, for the output of
 \code{flatten}.
 \[
 \begin{array}{lcl}
-\Exp &::=& \ldots \mid (\key{vector}\, \Exp^{+}) \\
+\Exp &::=& \ldots \mid (\key{vector}\, \Arg^{+}) \\
    &\mid & (\key{vector-ref}\, \Arg\, \Int) \\
    &\mid & (\key{vector-set!}\,\Arg\,\Int\,\Arg)
 \end{array}