Fix 'definition equations' -> 'declaration equations'

(Only tangentially related to the PR, but close enough.)

chapters/classes.tex

@@ -469,8 +469,7 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 that will ensure that any discrete-valued expression has at most discrete-time variability, see \cref{variability-of-expressions}.
 \end{nonnormative}
 
-The variability of expressions and restrictions on variability for
-definition equations is given in \cref{variability-of-expressions}.
+The variability of expressions and restrictions on variability for declaration equations is given in \cref{variability-of-expressions}.
 
 \begin{nonnormative}
 Note that discrete-time \emph{expressions} include parameter expressions, whereas discrete-time \emph{variables} do not include parameter variables.