[skip ci] Fix typo in create index design doc (#11437)

Signed-off-by: NEdward Zeng <jie.zeng@zilliz.com>

docs/design_docs/milvus_create_index_en.md

@@ -224,7 +224,7 @@ message SegmentInfo {
 
 ![data_coord_flushed](./graphs/milvus_create_index_root_coord_check.png)
 
-19. There is a backgroud service, `checkFlushedSegmentLoop`, in `RootCoord`. `checkFlushedSegmentLoop` would periodically check whether there is a segment that needs to be created index but has not been created, the default interval is `10 minutes`, and call `DataCoord` and `IndexCoord`'s service to create index on these segments.
+19. There is a background service, `checkFlushedSegmentLoop`, in `RootCoord`. `checkFlushedSegmentLoop` would periodically check whether there is a segment that needs to be created index but has not been created, the default interval is `10 minutes`, and call `DataCoord` and `IndexCoord`'s service to create index on these segments.
 
 20. In `Milvus 2.0`, `Create Index` is an asynchronous operation, so the `SDK` needs to send `GetIndexStates` request to `IndexCoord` periodically to check if the index has been created, the `proto` is defined as follow.