fix fig ref

book.tex

@@ -3528,7 +3528,7 @@ conditional control flow.
 \label{sec:r2-lang}
 
 The concrete syntax of the $R_2$ language is defined in
-Figure~\ref{fig:r2-concretesyntax} and the abstract syntax is defined
+Figure~\ref{fig:r2-concrete-syntax} and the abstract syntax is defined
 in Figure~\ref{fig:r2-syntax}. The $R_2$ language includes all of
 $R_1$ (shown in gray), the Boolean literals \code{\#t} and \code{\#f},
 and the conditional \code{if} expression. Also, we expand the