fix README example to mention `--grouping`
diff --git a/README.md b/README.md
index 4602546..3260be7 100644
--- a/README.md
+++ b/README.md
@@ -246,10 +246,11 @@
Libraries can be grouped using regular expressions. You can
-specify what regular expressions to use by providing a `grouping.yaml` file:
+specify what regular expressions to use by providing a `grouping.yaml` file
+with the `--grouping` flag:
```console
-$ dart2js_info library_size out.js.info.data grouping.yaml
+$ dart2js_info library_size out.js.info.data --grouping grouping.yaml
```
The format of the `grouping.yaml` file is as follows:
