minor symfony#14991 [Validator] Use single quotes for a string (wkania)

This PR was merged into the 4.4 branch.

Discussion
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[Validator] Use single quotes for a string

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Commits
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f085b32 [Validator] Use single quotes for a string

Author: OskarStark

validation/groups.rst

@@ -123,7 +123,7 @@ user registers and when a user updates their contact information later:
                 ]));
 
                 $metadata->addPropertyConstraint('city', new Assert\Length([
-                    "min" => 2,
+                    'min' => 2,
                 ]));
             }
         }