minor symfony#7281 PHP sample had a mistake (manoakys)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes symfony#7281).

Discussion
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PHP sample had a mistake

was $metadata->addPropertyConstraint('age', new Assert\NotEqualTo('Mary'));
should be $metadata->addPropertyConstraint('firstName', new Assert\NotEqualTo('Mary'));

Commits
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6781134 PHP sample had mistake

Author: xabbuh

reference/constraints/NotEqualTo.rst

@@ -102,7 +102,7 @@ the following:
         {
             public static function loadValidatorMetadata(ClassMetadata $metadata)
             {
-                $metadata->addPropertyConstraint('age', new Assert\NotEqualTo('Mary'));
+                $metadata->addPropertyConstraint('firstName', new Assert\NotEqualTo('Mary'));
 
                 $metadata->addPropertyConstraint('age', new Assert\NotEqualTo(array(
                     'value' => 15,