Update chapters/classes.tex

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

chapters/classes.tex

@@ -1397,7 +1397,7 @@ \section{Predefined Types and Classes}\label{predefined-types-and-classes}
 The definitions use \lstinline!RealType!\indexinline{RealType}, \lstinline!IntegerType!\indexinline{IntegerType}, \lstinline!BooleanType!\indexinline{BooleanType}, \lstinline!StringType!\indexinline{StringType}, \lstinline!EnumType!\indexinline{EnumType} as mnemonics corresponding to machine representations.
 These are called the \firstuse[primitive type]{primitive types}.
 
-\begin{definition}[Fall-back value]\label{def:fall-back-value}\index{fall-back value}
+\begin{definition}[Fallback value]\label{def:fallback-value}\index{fallback value}
 When the start-value is needed and the \lstinline!start!-attribute is not provided, the fall-back value is used instead.
 The fall-back value can be be used as guess-value in system of equations and when initializing variables in algorithm sections.
 The fall-back value for variables of the different predefined types are defined below.