report_stages: Fix invocation of som_alerts_dispatcher

BUG=None
TEST=report_stages_unittest

Change-Id: Iaacb5c8eeb462cfdbe25ec5cc25701f6766e9993
Reviewed-on: https://chromium-review.googlesource.com/465506
Commit-Ready: David Riley <davidriley@chromium.org>
Tested-by: David Riley <davidriley@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/cbuildbot/stages/report_stages.py b/cbuildbot/stages/report_stages.py
index 69db244..92f8ff9 100644
--- a/cbuildbot/stages/report_stages.py
+++ b/cbuildbot/stages/report_stages.py
@@ -908,10 +908,11 @@
 
     Args:
       db_credentials_dir: Path to CIDB database credentials.
+      tree: Sheriff-o-Matic tree to submit alerts to.
     """
     dispatcher_cmd = [os.path.join(self._build_root, 'chromite', 'scripts',
-                                   'som_alerts_dispatcher',
-                                   '--som_tree', tree)]
+                                   'som_alerts_dispatcher'),
+                      '--som_tree', tree]
     if buildbucket_lib.GetServiceAccount(constants.CHROMEOS_SERVICE_ACCOUNT):
       # User the service account file if it exists.
       dispatcher_cmd.extend(['--service_acct_json',