minor #11760 Update Collection.rst (murtukov)

This PR was submitted for the 4.3 branch but it was merged into the 3.4 branch instead (closes #11760).

Discussion
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Update Collection.rst

The line:
**`personal_email: Email`**
contains a mistake, as every constraint name should start on a new line and with the - symbol.
If I use the code sample in the yaml file, it would throw an exception:
`The value Email is not an instance of Constraint in constraint Symfony\Component\Validator\Constraints\Required"`

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Commits
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1440ca3 Update Collection.rst

Author: javiereguiluz

reference/constraints/Collection.rst

@@ -94,7 +94,8 @@ following:
                 profileData:
                     - Collection:
                         fields:
-                            personal_email: Email
+                            personal_email: 
+                                - Email: ~
                             short_bio:
                                 - NotBlank: ~
                                 - Length: