fix typo

roles/custom/matrix-dendrite/templates/dendrite/dendrite.yaml.j2

@@ -296,7 +296,7 @@ media_api:
   dynamic_thumbnails: {{ matrix_dendrite_media_api_dynamic_thumbnails|to_json }}
 
   # The maximum number of simultaneous thumbnail generators to run.
-  max_thumbnail_generators: {{ matrix_dendrite_media_api_max_thumbnails_generators|to_json }}
+  max_thumbnail_generators: {{ matrix_dendrite_media_api_max_thumbnail_generators|to_json }}
 
   # A list of thumbnail sizes to be generated for media content.
   thumbnail_sizes: