Update async-checkpoint.md (#1155)

fix the mistake of "manager"

docs/design/async-checkpoint.md

@@ -95,13 +95,13 @@ implement the checkpointing process.
 <img src="../figures/async-ckpt-classes.jpg" alt="Async Checkpoint Classes" width="1000">
 </div>
 
-- **AgentCkptManger**
+- **AgentCkptManager**
  - One instance runs in each agent process.
  memory and the storage.
  - Get the Shared lock of shared memory and save the checkpoint state into the storage.
  - One of Agent check if all agents finish the writing and commit the checkpoint.
 
-- **TrainCkptManger**
+- **TrainCkptManager**
  - One instance runs in each training process.
  - Is responsible for coping the checkpointing state from GPU to shared memory.
  - Notifies the AgentCkptManger to save the checkpoint state into the storage.