Update constraints.md (#5172)

Hi.
This proposal of modification involves adding a dash to the
"foreign_key" type of constraint, since I'm under the impression that it
shoud be a separate list element and not another "primary_key" property.
Regards.
IL.

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website/docs/reference/resource-properties/constraints.md

@@ -36,7 +36,7 @@ models:
     constraints:
       - type: primary_key
         columns: [<first_column>, <second_column>, ...]
-        type: foreign_key # multi_column
+      - type: foreign_key # multi_column
         columns: [<first_column>, <second_column>, ...]
         expression: "<other_model_schema>.<other_model_name> (<other_model_first_column>, <other_model_second_column>, ...)"
       - type: check