Fix typo in trace

Thanks to Xiaolong Liu
https://stacks.math.columbia.edu/tag/03TY#comment-8300

trace.tex

@@ -1105,7 +1105,7 @@ \section{Lefschetz numbers}
 have seen in Section \ref{section-frobenii}, the Frobenius $\pi_X$ acts on
 $K_{\bar x}$. The {\it local Lefschetz number} of $K$ is the sum
 $$
-\sum\nolimits_{x\in X(k)} \text{Tr}(\pi_X |_{K_{\overline{x}}})
+\sum\nolimits_{x\in X(k)} \text{Tr}(\pi_x |_{K_{\overline{x}}})
 $$
 which is again an element of $\Lambda^\natural$.
 \end{definition}