[mod] searxng_extras - minor improvements

- fix docs/searxng_extra/standalone_searx.py.rst
- add SPDX tag
- pylint standalone_searx.py and update_wikidata_units.py

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searxng_extra/update/update_engine_descriptions.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python
+# SPDX-License-Identifier: AGPL-3.0-or-later
 
 import json
 from urllib.parse import urlparse