Adding missing comma in spec list.

Fixing #2046, syntax error caused by missing comma on CONTAINER_SPEC_KWARGS list.
Signed-off-by: Chris Mark <chrismarkou92@gmail.com>

docker/models/services.py

@@ -276,7 +276,7 @@ CONTAINER_SPEC_KWARGS = [
     'labels',
     'mounts',
     'open_stdin',
-    'privileges'
+    'privileges',
     'read_only',
     'secrets',
     'stop_grace_period',