[skip ci]Update building README.md (#8774)

Signed-off-by: NWeida Zhu <weida.zhu@zilliz.com>

build/README.md

@@ -64,7 +64,7 @@ The following scripts are found in the [`build/`](.) directory. Note that all sc
   * `build/builder.sh make unittest`: Run all unit tests
   * `build/builder.sh make clean`: Clean up all the generated files
 
-You can specify a different OS for builder by setting `OS_NAME` which defaults to `ubuntu18.04`. Valid OS name are `ubuntu18.04`, `centos7`.
+You can specify different OS for builder by setting `OS_NAME` which defaults to `ubuntu18.04`. Valid OS are `ubuntu18.04`, `centos7`.
 
 To specify `centos7` builder, use these command: