replace tabs by spaces in the code owners file

The contributing instructions in the README are not in the GitHub contributing
guide, which means that people coming from the GitHub issues are less likely to
see them.

The template was not grammatical to me as it was. I have edited the language a tiny bit to make the close comment more clear and less abrasive.

* github: config for probot-stale bot

* github: use stale label, instead of wontfix