bug #5450 Fixing "Undefined method" error in code example (nebkam)

This PR was submitted for the 2.7 branch but it was merged into the 2.6 branch instead (closes #5450).

Discussion
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Fixing "Undefined method" error in code example

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.7
| Fixed tickets | -

The code example tries to call `getToken` on `Symfony\Component\Security\Core\Authorization\AuthorizationChecker`, which fails on Symfony 2.7 because the method is undefined.
This change utilizes the TokenStorage class, introduced in Symfony 2.6

Commits
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fa3936b Fixing "Undefined method" error on Symfony 2.7

Author: xabbuh

cookbook/security/impersonating_user.rst

@@ -93,9 +93,10 @@ over the user's roles until you find one that a ``SwitchUserRole`` object::
     use Symfony\Component\Security\Core\Role\SwitchUserRole;
 
     $authChecker = $this->get('security.authorization_checker');
+    $tokenStorage = $this->get('security.token_storage');
 
     if ($authChecker->isGranted('ROLE_PREVIOUS_ADMIN')) {
-        foreach ($authChecker->getToken()->getRoles() as $role) {
+        foreach ($tokenStorage->getToken()->getRoles() as $role) {
             if ($role instanceof SwitchUserRole) {
                 $impersonatingUser = $role->getSource()->getUser();
                 break;