minor symfony#9696 Update entity_provider.rst (matt9mg, javiereguiluz)

This PR was submitted for the 4.0 branch but it was merged into the 2.8 branch instead (closes symfony#9696).

Discussion
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Update entity_provider.rst

Keep the array syntax consistant between the 2 serialization functions that are implemented

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Commits
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2ed5e7d Fixed inconsistency differently
4ea991d Update entity_provider.rst

security/entity_provider.rst

@@ -133,7 +133,7 @@ For this entry, suppose that you already have a ``User`` entity inside an
                 $this->password,
                 // see section on salt below
                 // $this->salt
-            ) = unserialize($serialized, ['allowed_classes' => false]);
+            ) = unserialize($serialized, array('allowed_classes' => false));
         }
     }