Remove reference to WithExit in README
This function was removed from the containerd package in
@@ -166,7 +166,7 @@ If you have [criu](https://criu.org/Main_Page) installed on your machine you can
```go
// checkpoint the task then push it to a registry
-checkpoint, err := task.Checkpoint(context, containerd.WithExit)
+checkpoint, err := task.Checkpoint(context)
err := client.Push(context, "myregistry/checkpoints/redis:master", checkpoint)
