Correct KubernetesProvider.submit docstring, to not return None (#3507)

See issue #2949 for context

Author: benclifford

parsl/providers/kubernetes/kube.py

@@ -171,7 +171,6 @@ def submit(self, cmd_string, tasks_per_node, job_name="parsl"):
              - job_name (String): Name for job, must be unique
 
         Returns:
-             - None: At capacity, cannot provision more
              - job_id: (string) Identifier for the job
         """