minor symfony#9626 FIX : accessing global variable tokenStorage (luxferoo)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#9626).

Discussion
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FIX : accessing global variable tokenStorage

Global variable tokenStorage needs '$this->' to get accessed correctly

Commits
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42d7bfd FIX : accessing global variable tokenStorage

Author: javiereguiluz

session/proxy_examples.rst

@@ -134,7 +134,7 @@ can intercept the session before it is written::
 
         private function getUser()
         {
-            if (!$token = $tokenStorage->getToken()) {
+            if (!$token = $this->tokenStorage->getToken()) {
                 return;
             }