Fix typo in proof Lemma 0ABI

THanks to 羽山籍真
https://stacks.math.columbia.edu/tag/0ABI#comment-6718

more-groupoids.tex

@@ -2928,7 +2928,7 @@ \section{Finite groupoids}
 Hence there exists a point $r_1 \in R$ with $s(r_1) = u_1$
 such that $h$ is zero in $t(r_1)$. Since $s$ is finite we
 can find a specialization $r_1 \leadsto r_2$ with $s(r_2) = u_2$.
-However, then we conclude that $f$ is zero in $u'_2 = t(r_2)$
+However, then we conclude that $h$ is zero in $u'_2 = t(r_2)$
 which contradicts the fact that $j^{-1}D(h) \cap Z$
 is $R$-invariant and $u_2$ is in it. Thus $V$ is open.