Fix wrong distance [closes #34]

chapter29.tex

@@ -196,7 +196,7 @@ between points $(4,2)$ and $(3,-1)$:
 \begin{lstlisting}
 P a = {4,2};
 P b = {3,-1};
-cout << abs(b-a) << "\n"; // 3.60555
+cout << abs(b-a) << "\n"; // 3.16228
 \end{lstlisting}
 
 The function \texttt{arg(v)} calculates the