[book] [controller] fixed the code of a session sample code

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

This code was updated in the past to use the $request controller
injection and the resulting code was a bit confusing. When you get
the attribute set by another controller, it's better to use a
different attribute name, to make it clear that it wasn't set at
this controller.