typo

book.tex

@@ -7678,7 +7678,7 @@ x86_3 &::= & (\key{program} \;\itm{info} \;\Def\ldots)
 \]
 \end{minipage}
 }
-\caption{The concrete syntax of x86$_3$ (extends x86$_2$ of Figure~\ref{fig:x86-2}).}
+\caption{The concrete syntax of x86$_3$ (extends x86$_2$ of Figure~\ref{fig:x86-2-concrete}).}
 \label{fig:x86-3-concrete}
 \end{figure}