minor #15038 Unique validator: Better description for the value placeholder (derrabus)

This PR was merged into the 4.4 branch.

Discussion
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Unique validator: Better description for the value placeholder

The current description does not match the actual behavior of the validator, as spotted in symfony/symfony#40305 by @UFTimmy.

Commits
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00fbbdc Unique validator: Better description for the value placeholder

Author: javiereguiluz

reference/constraints/Unique.rst

@@ -107,7 +107,7 @@ You can use the following parameters in this message:
 =============================  ================================================
 Parameter                      Description
 =============================  ================================================
-``{{ value }}``                The repeated value
+``{{ value }}``                The current (invalid) value
 =============================  ================================================
 
 .. include:: /reference/constraints/_payload-option.rst.inc