minor #11539 [Session] Fix local variable in session example (Einenlum)

This PR was merged into the 4.2 branch.

Discussion
----------

[Session] Fix local variable in session example

Fixes this code example: https://symfony.com/doc/current/session.html#basic-usage
(`$session` -> `$this->session`)

Commits
-------

95fd311 Fix local variable in session example

Author: javiereguiluz

session.rst

@@ -146,13 +146,13 @@ controllers if you type-hint an argument with
         public function someMethod()
         {
             // stores an attribute in the session for later reuse
-            $session->set('attribute-name', 'attribute-value');
+            $this->session->set('attribute-name', 'attribute-value');
 
             // gets an attribute by name
-            $foo = $session->get('foo');
+            $foo = $this->session->get('foo');
 
             // the second argument is the value returned when the attribute doesn't exist
-            $filters = $session->get('filters', []);
+            $filters = $this->session->get('filters', []);
 
             // ...
         }