fix typo in UnparsedSourceDefinition.__post_serialize_ (#4349)

* fix typo in UnparsedSourceDefinition.__post_serialize_

fix typo in UnparsedSourceDefinition.__post_serialize_

* update CHANGELOG.md

update CHANGELOG.md

add #4349

* Update changelog

Co-authored-by: Jeremy Cohen <jeremy@dbtlabs.com>

automatic commit by git-black, original commits:
  0d320c5

core/dbt/contracts/graph/unparsed.py

@@ -277,7 +277,7 @@ def yaml_key(self) -> "str":
 
     def __post_serialize__(self, dct):
         dct = super().__post_serialize__(dct)
-        if 'freshness' not in dct and self.freshness is None:
+        if "freshness" not in dct and self.freshness is None:
             dct["freshness"] = None
         return dct