Add missing index

Thanks to Jonas Ehrhard
https://stacks.math.columbia.edu/tag/0531#comment-6603

more-algebra.tex

@@ -4476,7 +4476,7 @@ \section{Descent of flatness along integral maps}
 \begin{proof}
 Let $x_1, \ldots, x_n \in S$ be generators of $S$ over $R$.
 For each $i$ we can choose a monic polynomial $P_i(T) \in R[T]$
-such that $P(x_i) = 0$ in $S$, see
+such that $P_i(x_i) = 0$ in $S$, see
 Algebra, Lemma \ref{algebra-lemma-finite-is-integral}.
 Say $\deg(P_i) = d_i$. By
 Lemma \ref{lemma-adjoin-one-root}