Fix typo in etale-cohomology.tex

Thanks to yogesh more
http://stacks.math.columbia.edu/tag/03PF#comment-1340

etale-cohomology.tex

@@ -2705,7 +2705,7 @@ \section{\'Etale coverings}
 for some affine opens $V_{ij} \subset U_i$. Since $\varphi_i$ is open
 (as \'etale morphisms are open), we see that
 $V = \bigcup_{i\in I} \bigcup_{j \in J_i} \varphi_i(V_{ij})$
-is an open covering of $U$.
+is an open covering of $V$.
 Further, since $V$ is quasi-compact, this covering has a finite
 refinement.
 \end{proof}