Update Chromium recipe instructions to reflect new flow (#9919)

dev/bots/README.md

@@ -61,8 +61,8 @@ The typical cycle for editing a recipe is:
 
 1. Make your edits (probably to files in
    `//chrome_infra/build/scripts/slave/recipes/flutter`).
-2. Run `build/scripts/slave/recipes.py simulation_test train flutter` to update
-   expected files  (remove the flutter if you need to do a global update).
+2. Run `build/scripts/slave/recipes.py --use-bootstrap test train` to
+   update expected files
 3. Run `build/scripts/tools/run_recipe.py flutter/<repo> slavename=<slavename>
    mastername=client.flutter buildername=<buildername>` where `<repo>` is one
    of `flutter` or `engine`, and `slavename` and `buildername` can be looked up