Fix typo

Assuming that's a typo since the entailment fails otherwise.

content/counterfactuals/introduction/strict-conditional.tex

@@ -99,7 +99,7 @@
 \begin{prob}
   Give proofs in \Log{S5} of:
   \begin{enumerate}
-    \item  $\Box \lnot !B \Entails !A \strictif !B$
+    \item  $\Box \lnot !A \Entails !A \strictif !B$
     \item $!A \strictif !B \Entails \Box(!A \strictif !B)$
     \item $\lnot(!A \strictif !B)  \Entails \Box\lnot(!A \strictif !B)$
   \end{enumerate}