Fix typo in document docker_swarm_service

mount property doesn't have property mode, but type

##### ISSUE TYPE
- Docs Pull Request

+label: docsite_pr

lib/ansible/modules/cloud/docker/docker_swarm_service.py

@@ -331,7 +331,7 @@
       driver_config:
         description:
           - Volume driver configuration.
-          - Can only be used when I(mode) is C(volume).
+          - Can only be used when I(type) is C(volume).
         suboptions:
           name:
             description: