Fix mistake in proof 09GK

Thanks to Herman Rohrbach
https://stacks.math.columbia.edu/tag/09GK#comment-2950

fields.tex

@@ -747,7 +747,7 @@ \section{Algebraic extensions}
 is finite (details omitted). Moreover, the fact that $E$ is algebraic
 over $F$ implies that $E = \bigcup_{P \in S} r(P, E)$.
 It is clear that $S$ has cardinality bounded by $\max(\aleph_0, |F|)$
-because the cardinality of a finite product of copies of $F$ has
+because the cardinality of a countable product of copies of $F$ has
 cardinality at most $\max(\aleph_0, |F|)$.
 Thus so does $E$.
 \end{proof}