Fix typo in setup.cfg (#12935)

```
airflow.api.connextion.openaip=*.yaml
```
to
```
airflow.api_connexion.openapi=*.yaml
```

setup.cfg

@@ -153,7 +153,7 @@ airflow=
     customized_form_field_behaviours.schema.json
     provider.yaml.schema.json
 
-airflow.api.connextion.openaip=*.yaml
+airflow.api_connexion.openapi=*.yaml
 airflow.serialization=*.json
 
 [options.entry_points]