[skip ci]Fix typo issue (#11121)

Signed-off-by: Nyanliang567 <yanliang.qiao@zilliz.com>

docs/design_docs/milvus_flush_collections_en.md

@@ -144,7 +144,7 @@ message DataNodeTtMsg {
 }
  ```
 
-8. There is a backgroud service, `startDataNodeTsLoop`, in `DataCoord` to process the message of `DataNodeTtMsg`.
+8. There is a background service, `startDataNodeTsLoop`, in `DataCoord` to process the message of `DataNodeTtMsg`.
     - Firstly, `DataCoord` would extract `channel_name` from `DataNodeTtMsg`, and filter out all sealed segments that are attached on this `channel_name`
     - Compare the timestamp when the segment enters into state of `Sealed` with the `DataNodeTtMsg.timestamp`, if `DataNodeTtMsg.timestamp` is greater, which means that all `ID`s belonging to that segment have been consumed by `DataNode`, it's safe to notify `DataNode` to write that segment into persistent storage. The `proto` is defined as follows:
 ```proto