Fix typo 'inde' -> 'index'

Addressing commend by Hans.

chapters/introduction.tex

@@ -123,7 +123,7 @@ \section{Notation}\label{notation}
 It is common to mix Modelica code with mathematical notation.
 For example, \lstinline!average($x$, $y$)! could be defined as $\frac{x + y}{2}$.
 
-\begin{definition}[Something]% Do not add this one to the inde!
+\begin{definition}[Something]% Do not add this one to the index!
 Text defining the meaning of \emph{something}.
 \end{definition}