[skip ci]Fix doc error in design doc (#11158)

Signed-off-by: NJackLCL <chenglong.li@zilliz.com>

docs/design_docs/milvus_create_index_en.md

@@ -88,7 +88,7 @@ type CreateIndexReqTask struct {
 }
 ```
 
-5. According to the index type and index parameters, `RootCoord` lists all the `Segments` that need to be indexed on this `Collection`. `RootCoord` would only check those `Segments` which have been flushed at this stage. We will describe how to deal with those newly add segments and growing segments later.
+5. According to the index type and index parameters, `RootCoord` lists all the `Segments` that need to be indexed on this `Collection`. `RootCoord` would only check those `Segments` which have been flushed at this stage. We will describe how to deal with those newly added segments and growing segments later.
 
 6. For each `Segment`, `RootCoord` would start a `Grpc` request to `DataCoord` to get `Binlog` paths of that `Segment`, the `proto` is defined as following