minor #18508 [Validator] Update Unique.rst (gregberger)

This PR was submitted for the 6.3 branch but it was merged into the 6.2 branch instead.

Discussion
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[Validator] Update Unique.rst

Typo in Attribute `fields` code example

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Commits
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0612308 Update Unique.rst

Author: javiereguiluz

reference/constraints/Unique.rst

@@ -120,7 +120,7 @@ collection::
 
         class Poi
         {
-            #[Assert\Unique(fields=['latitude', 'longitude'])]
+            #[Assert\Unique(fields: ['latitude', 'longitude'])]
             protected array $coordinates;
         }