Fix typo in bootstrap

THanks to F. Liu
https://stacks.math.columbia.edu/tag/046L#comment-7110

bootstrap.tex

@@ -1186,7 +1186,7 @@ \section{Slicing equivalence relations}
 Step 2 and denote $R'_u$ the restriction of $R$ to $U'_u$.
 Denote $h_u = s \circ \text{pr}_1 : U'_u \times_{g_u, U, t} R \to U$. Set
 $U' = \coprod_{u \in U} U'_u$, and $g = \coprod g_u$. Let $R'$ be the
-restriction of $R$ to $U$ as above. We claim that
+restriction of $R$ to $U'$ as above. We claim that
 the pair $(U', g)$ works\footnote{Here we should check that $U'$ is not
 too large, i.e., that it is isomorphic to an object of the category
 $\Sch_{fppf}$, see