missing dash in the sudo command

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] I have a fix to the document that I can paste below including example: input and output.
the following command: "$ sudo su stack" should be "$ sudo su - stack" to make sure that chnage from "root" home to "stack" home.

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Release: on 2018-10-20 14:00
SHA: f22bd0b89bffbb8b93ed6d9d0057cb392943d7cc
Source: https://git.openstack.org/cgit/openstack/kuryr-kubernetes/tree/doc/source/installation/devstack/basic.rst
URL: https://docs.openstack.org/kuryr-kubernetes/latest/installation/devstack/basic.html