Improve/change example 6.12 on linearization [linsys]

[MSchrau]

In paragraph 6.4 about linearization in example 6.12 an example is given that tries to show a linearization of the cruise control model. However, on page 6-31 it is said: ' Therefore we cannot formally linearize the full model.'. If that is true, then a bad example is used. Please consider, using a simpler example that shows how a linearization is done.

[Richard Murray]

Fixed in commit 9ce434f. Added a simplified example (cruise control) before the more complicated one.