Update chapters/classes.tex

Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/classes.tex

@@ -383,7 +383,7 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 definition equations is given in \cref{variability-of-expressions}.
 
 \begin{nonnormative}
-Note that for expressions discrete-time expressions include parameter expressions, whereas discrete-time variables do not include parameter variables.
+Note that discrete-time \emph{expressions} include parameter expressions, whereas discrete-time \emph{variables} do not include parameter variables.
 This is consistent with restrictions on variability for expressions normally being upper limits.
 
 For \lstinline!Real! variables we can distinguish two subtly different categories: discrete-time and piecewise constant, where the discrete-time variables are a subset of all piecewise constant variables.