Add 'time' to document index

chapters/operatorsandexpressions.tex

@@ -358,7 +358,7 @@ \subsection{Member Access Operator}\label{member-access-operator}
 Another use of dot notation: local classes which are members of a class can of course also be accessed using dot notation on the name of the class, not on instances of the class.
 \end{example}
 
-\subsection{Built-in Variable time}\label{built-in-variable-time}
+\subsection{Built-in Variable time}\label{built-in-variable-time}\indexinline{time}
 
 All declared variables are functions of the independent variable \lstinline!time!.
 The variable \lstinline!time! is a built-in variable available in all models and blocks, which is treated as an input variable.