Update guard_authentication.rst

Author: orangevinz

security/guard_authentication.rst

@@ -38,7 +38,7 @@ directly to your ``User`` class (the ``make:entity`` command is a good way to do
         // ...
 
     +     /**
-    +      * @ORM\Column(type="string", unique=true)
+    +      * @ORM\Column(type="string", unique=true, nullable=true)
     +      */
     +     private $apiToken;