fix: [stix2 import] Using the AS value parsing method for an AS value…

… that was missing it

misp_stix_converter/stix2misp/converters/stix2_observable_converter.py

@@ -532,7 +532,8 @@ def _parse_asn_observable(
             observed_data_id: Optional[str] = None) -> Iterator:
         object_id = getattr(observable, 'id', observed_data_id)
         yield from self._populate_object_attributes(
-            self._mapping.asn_attribute(), f'AS{observable.number}', object_id
+            self._mapping.asn_attribute(),
+            self._parse_AS_value(observable.number), object_id
         )
         if hasattr(observable, 'name'):
             yield from self._populate_object_attributes(