Turn 'y' in example to discrete-time variable

As suggested by Hans.

Co-authored-by: Hans Olsson <HansOlsson@users.noreply.github.com>

chapters/dae.tex

@@ -97,14 +97,13 @@ \chapter{Modelica DAE Representation}\label{modelica-dae-representation}
 \end{lstlisting}
 
 Another way that a discrete-time \lstinline!Real! variable can end up becoming determined by a nonlinear equation is through coupling with other variables.
-The other variables may be discrete-time, or continuous-time as in the following example:
 \begin{lstlisting}[language=modelica]
 model M
   discrete Real x(start = 1.0, fixed = true);
-  Real y;
+  discrete Real y(start=0.0, fixed=true);
 equation
-  y = x ^ 2 + 2 * exp(-time);
   when sample(1.0, 1.0) then
+    y = x ^ 2 + 2 * exp(-time);
     x = 3 * pre(x) - y; // OK, forming nonlinear equation system with y.
   end when;
 end M;