Merge pull request #3390 from modelica/eshmoylova-a-to-an

Update classes.tex

Author: HansOlsson

chapters/classes.tex

@@ -429,7 +429,7 @@ \section{Component Variability}\label{component-variability}
   This is called a \firstuse[---]{constant}, or \firstuse[constant!variable]{constant variable}\index{component variability!constant}.
   For further details, see \ref{constants}.
 \item
-  A variable \lstinline!ep! is called a \firstuse[parameter!evaluable]{evaluable parameter variable}\index{component variability!evaluable parameter} if all of the following applies:
+  A variable \lstinline!ep! is called an \firstuse[parameter!evaluable]{evaluable parameter variable}\index{component variability!evaluable parameter} if all of the following applies:
   \begin{itemize}
   \item
     It is declared with the \lstinline!parameter!\indexinline{parameter} prefix.