minor #11655 fix constraint reference YAML config syntax (xabbuh)

This PR was merged into the 4.3 branch.

Discussion
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fix constraint reference YAML config syntax

fixes symfony/symfony#31829

Commits
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73c26f6 fix constraint reference YAML config syntax

Author: javiereguiluz

reference/constraints/Negative.rst

@@ -47,7 +47,7 @@ The following constraint ensures that the ``withdraw`` of a  bank account
         App\Entity\TransferItem:
             properties:
                 withdraw:
-                    - Negative
+                    - Negative: ~
 
     .. code-block:: xml
 

reference/constraints/NegativeOrZero.rst

@@ -46,7 +46,7 @@ is a negative number or equal to zero:
         App\Entity\UnderGroundGarage:
             properties:
                 level:
-                    - NegativeOrZero
+                    - NegativeOrZero: ~
 
     .. code-block:: xml
 

reference/constraints/Positive.rst

@@ -47,7 +47,7 @@ positive number (greater than zero):
         App\Entity\Employee:
             properties:
                 income:
-                    - Positive
+                    - Positive: ~
 
     .. code-block:: xml
 

reference/constraints/PositiveOrZero.rst

@@ -46,7 +46,7 @@ is positive or zero:
         App\Entity\Person:
             properties:
                 siblings:
-                    - PositiveOrZero
+                    - PositiveOrZero: ~
 
     .. code-block:: xml