DM_dump/load/sync_process_exists_with_error 告警拼写错误

File: [/zh/handle-alerts.md](https://docs.pingcap.com/zh/tidb-data-migration/stable/handle-alerts)
```
DM_dump_process_exists_with_error
DM_load_process_exists_with_error
DM_sync_process_exists_with_error
```
这三个错误名字，exists应该更正为exits，prometheus的dm_worker.rules.yml中名字也需要改