Wrong ring in algebra

Thanks to nkym
https://stacks.math.columbia.edu/tag/03GE#comment-7689

algebra.tex

@@ -40126,7 +40126,7 @@ \section{Integral closure and smooth base change}
 $R[x]/(f)$ in $B[x]/(f)$. Suppose that $a \in A''$. It suffices
 to show that $a$ is in $S \otimes_R A$. By
 Lemma \ref{lemma-trick} we see that $f' a = \sum a_i x^i$ with $a_i \in A$.
-Since $f'$ is invertible in $B[x]_g/(f)$ (by definition of a standard
+Since $f'$ is invertible in $S$ (by definition of a standard
 \'etale ring map) we conclude that $a \in S \otimes_R A$ as desired.
 \end{proof}