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"

Author: murtukov

reference/constraints/Collection.rst

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