Update dendrite.yaml.j2

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

@@ -293,10 +293,10 @@ media_api:
   max_file_size_bytes: {{ matrix_dendrite_max_file_size_bytes|to_json }}
 
   # Whether to dynamically generate thumbnails if needed.
-  dynamic_thumbnails: {{ matrix_dendrite_mediaapi_dynamic_thumbnails }}
+  dynamic_thumbnails: {{ matrix_dendrite_mediaapi_dynamic_thumbnails|to_json }}
 
   # The maximum number of simultaneous thumbnail generators to run.
-  max_thumbnail_generators: {{ matrix_dendrite_mediaapi_max_thumbnails_generators }}
+  max_thumbnail_generators: {{ matrix_dendrite_mediaapi_max_thumbnails_generators|to_json }}
 
   # A list of thumbnail sizes to be generated for media content.
   thumbnail_sizes: