Fix phrasing

It's the formulas that are valid, not the validity statements.

Author: eguindon

content/normal-modal-logic/syntax-and-semantics/modal-validity.tex

@@ -61,9 +61,9 @@
 \begin{prob}
   Show that the following are valid:
   \begin{enumerate}
-  \item $\Entails \Box p \lif \Box (q \lif p)$;
-  \item $\Entails \Box \lnot \lfalse$;
-  \item $\Entails \Box p \lif (\Box q \lif \Box p)$.
+  \item $\Box p \lif \Box (q \lif p)$;
+  \item $\Box \lnot \lfalse$;
+  \item $\Box p \lif (\Box q \lif \Box p)$.
   \end{enumerate}
 \end{prob}