Update worker type and description

.arkindex.yml

@@ -4,7 +4,7 @@ version: 2
 workers:
   - slug: thumbnails-generator
     name: Thumbnails Generator
-    type: generator
+    type: Utils
     docker:
       build: Dockerfile
     configuration:

worker_thumbnails_generator/worker.py

@@ -127,7 +127,7 @@ class ThumbnailsGenerator(ElementsWorker):
 
 def main() -> None:
     ThumbnailsGenerator(
-        description="Worker to generate thumbnails for Arkindex elements"
+        description="Worker to generate thumbnails for Arkindex elements: Utility process"
     ).run()