Fixed typo.

src/topology/main/preimage.tex

@@ -35,7 +35,7 @@
     \begin{enumerate}[label=(PF\arabic*)]
         \item $P(\emptyset) = \emptyset$. 
         \item[(PF2')] For each $\mathcal{U} \subset \mathcal{B}$ and $V \in \mathcal{B}$ with $V \subset \bigcup_{U \in \mathcal{U}}U$, $p(V) \subset \bigcup_{U \in \mathcal{U}}p(U)$.  
-        \item[(PF3')] For each $U, V \subset \mathcal{B}$, $p(U) \cap p(V) \subset \bigcup_{W \in \mathcal{B}, W \subset U \cap V}p(W)$. 
+        \item[(PF3')] For each $U, V \in \mathcal{B}$, $p(U) \cap p(V) \subset \bigcup_{W \in \mathcal{B}, W \subset U \cap V}p(W)$. 
     \end{enumerate}
 \end{definition}