doc(README.md): shortening command

Author: arschles

_tests/README.md

@@ -10,8 +10,7 @@ for which we're working on a permanent fix (possible in [this GH issue](https://
 below are commands you can run to work around the failure:
 
 ```console
-$ kubectl exec -it deis-workflow-qoxhz -- sh
-/app # ./manage.py shell
+$ kubectl exec -it deis-workflow-qoxhz python manage.py shell
 Python 2.7.10 (default, Aug 13 2015, 12:27:27)
 [GCC 4.9.2] on linux2
 >>> from django.contrib.auth import get_user_model