chore(docs): remove .py from db migrate -m (apache#14331)

no need to state the python extension 
before: add_metadata_column_to_annotation_model.py.py
after: add_metadata_column_to_annotation_model.py

CONTRIBUTING.md

@@ -1137,7 +1137,7 @@ Submissions will be considered for submission (or removal) on a case-by-case bas
 1. Generate the migration file
 
    ```bash
-   superset db migrate -m 'add_metadata_column_to_annotation_model.py'
+   superset db migrate -m 'add_metadata_column_to_annotation_model'
    ```
 
    This will generate a file in `migrations/version/{SHA}_this_will_be_in_the_migration_filename.py`.