message: Complete a device string in configure_collective_ops document.
diff: @@ -606,7 +606,7 @@ class Context(object):
  Args:
  collective_leader: a device string for collective leader, e.g.
  - "/job:worker/replica:0/task:"; empty string means local execution of
  + "/job:worker/replica:0/task:0"; empty string means local execution of
  collective ops.
  scoped_allocator_enabled_ops: a tuple or a list of op names for scoped
  allocator to run with.
  