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

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.
minor #3735 [book] [controller] fixed the code of a session sample code (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
-------

9da7f9b [book] [controller] fixed the code of a session sample code