Fix the direction of an arrow

Thanks to Kien Nguyen
http://stacks.math.columbia.edu/tag/03N4#comment-1618

CONTRIBUTORS

@@ -119,6 +119,7 @@ Jackson Morrow
 Yusuf Mustopa
 David Mykytyn
 Josh Nichols-Barrer
+Kien Nguyen
 Thomas Nyberg
 Masahiro Ohno
 Catherine O'Neil

etale-cohomology.tex

@@ -154,10 +154,14 @@ \section{The \'etale topology}
 (resp.\ abelian group, vector space, etc) $\mathcal{F}(U)$,
 \item for each pair $U, \ U'$ of \'etale schemes over $X$,
 and each morphism $U \to U'$ over $X$ (which is
-automatically \'etale) a restriction map $\rho^{U}_{U'}
-: \mathcal{F}(U) \to \mathcal{F}(U')$
+automatically \'etale) a restriction map
+$\rho^{U'}_U : \mathcal{F}(U') \to \mathcal{F}(U)$
 \end{enumerate}
-These data have to satisfy the following {\it sheaf axiom}:
+These data have to satisfy the condition that $\rho^U_U = \text{id}$
+in case of the identity morphism $U \to U$
+and that $\rho^{U'}_U \circ \rho^{U''}_{U'} = \rho^{U''}_U$
+when we have morphisms $U \to U' \to U''$ of schemes \'etale over $X$
+as well as the following {\it sheaf axiom}:
 \begin{itemize}
 \item[$(*)$] for every \'etale covering $\{ \varphi_i : U_i \to X\}_{i \in
 I}$, the diagram