minor #4216 Update Collection.rst (azarzag)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4216).

Discussion
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Update Collection.rst

Fixes wrong key in php code block.

Commits
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bf529bb Update Collection.rst

reference/constraints/Collection.rst

@@ -144,7 +144,7 @@ blank but is no longer than 100 characters in length, you would do the following
                 $metadata->addPropertyConstraint('profileData', new Assert\Collection(array(
                     'fields' => array(
                         'personal_email' => new Assert\Email(),
-                        'lastName' => array(
+                        'short_bio' => array(
                             new Assert\NotBlank(),
                             new Assert\Length(array(
                                 'max' => 100,