Add a note about depot_tools to the `led` step (#96830)

dev/bots/README.md

@@ -95,6 +95,7 @@ The typical cycle for editing a recipe is:
 4. Run `led get-builder 'luci.flutter.prod:BUILDER_NAME' | led edit -p 'revision="GIT_HASH"' | led edit-recipe-bundle | led launch`, where `BUILDER_NAME` is the builder name (e.g. `Linux Engine`), and
    `GIT_HASH` is the hash to build (which is important for the engine but not
    for the framework).
+   * If `led` fails, ensure that your `depot_tools` checkout is up to date.
 5. To submit a CL, you need a local branch first (`git checkout -b [some branch name]`).
 6. Upload the patch (`git commit`, `git cl upload`), and open the outputted URL to the CL.
 7. Use "Find owners" to get reviewers for the CL