Add precise reference

Thanks to Aravind Asok
http://stacks.math.columbia.edu/tag/0991#comment-1850

proetale.tex

@@ -2706,8 +2706,9 @@ \section{Points of the pro-\'etale site}
 & S
 }
 $$
-with $U \to S$ weakly \'etale. In exactly the same manner as in the
-chapter on \'etale cohomology one shows that the category of pro-\'etale
+with $U \to S$ weakly \'etale. In exactly the same manner as in
+\'Etale Cohomology, Lemma \ref{etale-cohomology-lemma-cofinal-etale}
+one shows that the category of pro-\'etale
 neighbourhoods of $\overline{s}$ is cofiltered. Moreover, if
 $(U, \overline{u})$ is a pro-\'etale neighbourhood, and if $\{U_i \to U\}$
 is a pro-\'etale covering, then there exists an $i$ and a lift of