Simplify examples of invalid code

As suggested by Hans.

Co-authored-by: Hans Olsson <HansOlsson@users.noreply.github.com>

chapters/introduction.tex

@@ -107,8 +107,7 @@ \section{Notation}\label{notation}
 \begin{lstlisting}[language=modelica]
 model Example "Example used to illustrate syntax highlighting"
   /* The string above is a class description string, this is a comment. */
-//String s = 1.0; /* Commenting out invalid code like this is rare. */
-  /* Instead of commenting-out invalid code, it is typically done this way: */
+  /* Invalid code is typically done this way: */
   String s = 1.0; // Error: No conversion form Real to String.
   Real x;
 equation