Fix omission of of in algebra

Thanks to nhw
https://stacks.math.columbia.edu/tag/00U7#comment-8297

algebra.tex

@@ -38970,7 +38970,7 @@ \section{\'Etale ring maps}
 By Lemma \ref{lemma-etale-at-prime} the ring map
 $S'_{\mathfrak q'}/\mathfrak p S'_{\mathfrak q'} \to
 S_{\mathfrak q}/\mathfrak p S_{\mathfrak q}$
-is a map finite separable extensions of $\kappa(\mathfrak p)$.
+is a map of finite separable extensions of $\kappa(\mathfrak p)$.
 In particular it is flat. Hence by
 Lemma \ref{lemma-criterion-flatness-fibre} we see that
 $S'_{\mathfrak q'} \to S_{\mathfrak q}$ is flat. Thus $S' \to S$