minor symfony#12858 Update Valid.rst (artyuum)

This PR was submitted for the 5.0 branch but it was merged into the 3.4 branch instead (closes symfony#12858).

Discussion
----------

Update Valid.rst

Changed few double-quotes to single-quotes for code consistency

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

992f0d8 Update Valid.rst

Author: xabbuh

reference/constraints/Valid.rst

@@ -157,7 +157,7 @@ stores an ``Address`` instance in the ``$address`` property::
             {
                 $metadata->addPropertyConstraint('street', new Assert\NotBlank());
                 $metadata->addPropertyConstraint('zipCode', new Assert\NotBlank());
-                $metadata->addPropertyConstraint('zipCode', new Assert\Length(["max" => 5]));
+                $metadata->addPropertyConstraint('zipCode', new Assert\Length(['max' => 5]));
             }
         }
 
@@ -172,7 +172,7 @@ stores an ``Address`` instance in the ``$address`` property::
             public static function loadValidatorMetadata(ClassMetadata $metadata)
             {
                 $metadata->addPropertyConstraint('firstName', new Assert\NotBlank());
-                $metadata->addPropertyConstraint('firstName', new Assert\Length(["min" => 4]));
+                $metadata->addPropertyConstraint('firstName', new Assert\Length(['min' => 4]));
                 $metadata->addPropertyConstraint('lastName', new Assert\NotBlank());
             }
         }