minor #7758 docs: choice constraints reference (VEBERArnaud)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7758).

Discussion
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docs: choice constraints reference

## Description

fix wrong yaml example for Choice Constraints

Commits
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e109eab docs: choice constraints reference

reference/constraints/Choice.rst

@@ -62,7 +62,8 @@ If your valid choice list is simple, you can pass them in directly via the
         # src/AppBundle/Resources/config/validation.yml
         AppBundle\Entity\Author:
             properties:
-                city: [New York, Berlin, Tokyo]
+                city:
+                    - Choice: [New York, Berlin, Tokyo]
                 gender:
                     - Choice:
                         choices:  [male, female]