Change 'a discrete-time variables' -> 'a discrete-time variable'

Fixes #2699

chapters/equations.tex

@@ -798,7 +798,7 @@ \section{Initialization, initial equation, and initial algorithm}\label{initiali
   \item
     For a non-discrete-time (that is continuous-time) \lstinline!Real! variable \lstinline!vc!, the equation \lstinline!vc = startExpression! is added to the initialization equations.
   \item
-    For a discrete-time variables \lstinline!vd!, the equation \lstinline!pre(vd) = startExpression! is added to the initialization equations.
+    For a discrete-time variable \lstinline!vd!, the equation \lstinline!pre(vd) = startExpression! is added to the initialization equations.
   \item
     For a variable declared as \lstinline!constant! or \lstinline!parameter!, no equation is added to the initialization equations.
   \end{itemize}