minor symfony#10110 Undefined variable $userName in example (Trekels)

This PR was submitted for the 4.1 branch but it was merged into the 2.8 branch instead (closes symfony#10110).

Discussion
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Undefined variable $userName in example

The $username variable in the `WebServiceUserProvider` example is not set, since the user object must implement the `UserInterface` (according to best practice) we can set the variable by calling `$user->getUsername()`.

Commits
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e6d500f Undefined variable $userName in example

security/custom_provider.rst

@@ -144,6 +144,8 @@ Here's an example of how this might look::
                 );
             }
 
+            $username = $user->getUsername();
+
             return $this->fetchUser($username);
         }