Fix typo

Author: athomas

.cloud_build/specification/cloudbuild.yaml

@@ -2,7 +2,7 @@ steps:
 - name: 'ubuntu'
   args: ['specification/scripts/build_pdf']
 - name: 'gcr.io/cloud-builders/gsutil'
-  args: ['cp', '-m', 'r', 'specification/artifacts/', 'gs://dart-specification']
+  args: ['cp', '-m', '-r', 'specification/artifacts/', 'gs://dart-specification']
 options:
   automapSubstitutions: true
   logging: CLOUD_LOGGING_ONLY