[doc] Fix the missing factor 4 in the trace with gamma5 (#28)

doc/manual/metric.tex

@@ -59,7 +59,7 @@ \chapter{A few notes on the use of a metric}
 in the trace are 
 \begin{eqnarray}
 	Tr[\gamma_5\gamma_{m_1}\gamma_{m_2}\gamma_{m_3}\gamma_{m_4}] & = &
-		\epsilon_{\mu_1\mu_2\mu_3\mu_4} \nonumber
+		4 \epsilon_{\mu_1\mu_2\mu_3\mu_4} \nonumber
 \end{eqnarray}
 This has a number of interesting consequences. The V-A and V+A currents are 
 represented by $\gamma_7\gamma_\mu = (1-\gamma_5)\gamma_\mu$ and