fix title from assertion -> proposition rename

docs/ghost-state.tex

@@ -52,7 +52,7 @@ Persistence is preserved by conjunction, disjunction, separating conjunction as
 
 
 
-\subsection{Timeless Proposition and Except-0}
+\subsection{Timeless Propositions and Except-0}
 
 One of the troubles of working in a step-indexed logic is the ``later'' modality $\later$.
 It turns out that we can somewhat mitigate this trouble by working below the following \emph{except-0} modality: