Use open en-dash instead of open em-dash

This is how it's done on 'master'.

chapters/synchronous.tex

@@ -229,7 +229,7 @@ \subsection{Clocks and Clocked Variables}\label{clocks-and-clocked-variables}
   \begin{center}
     \includegraphics{clock}
   \end{center}
-  \caption{A clock variable.  The value of a clock variable is not defined --- the plot marks only indicate \emph{when} the clock is active.}\label{fig:clock-variable}
+  \caption{A clock variable.  The value of a clock variable is not defined -- the plot marks only indicate \emph{when} the clock is active.}\label{fig:clock-variable}
 \end{figure}
 
 \begin{definition}[Clocked variable]