minor symfony#7134 Update Choice.rst (SebLours)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes symfony#7134).

Discussion
----------

Update Choice.rst

Fixed city choice (php code block).

Commits
-------

70b66fa Update Choice.rst

Author: xabbuh

reference/constraints/Choice.rst

@@ -111,7 +111,7 @@ If your valid choice list is simple, you can pass them in directly via the
             public static function loadValidatorMetadata(ClassMetadata $metadata)
             {
                 $metadata->addPropertyConstraint(
-                    'gender',
+                    'city',
                      new Assert\Choice(array('New York', 'Berlin', 'Tokyo'))
                  );