Fix typo "leval" -> "legal"

chapters/classes.tex

@@ -1042,7 +1042,7 @@ \section{Balanced Models}\doublelabel{balanced-models}
 values of constants and parameters.
 
 \begin{nonnormative}
-Here, \emph{leval values} must respect final bindings and min/max-restrictions. A tool shall verify the \emph{locally balanced} property for the actual
+Here, \emph{legal values} must respect final bindings and min/max-restrictions.  A tool shall verify the \emph{locally balanced} property for the actual
 values of parameters and constants in the simulation model.  It is a quality of implementation for a tool to verify this property in general, due to
 arrays of (locally) undefined sizes, conditional declarations, for loops etc.
 \end{nonnormative}