Fix typo in README
Fix typo in README

In #311 the README was updated to update references to the change in default
branch name, as part of that change the badges were updated to remove the
obsolete badges for the old CI systems we no longer use and update it to use
a badge for github actions. However, there was a copy and paste error in that
link and it was referring to the wrong repository and not rendering correctly.
This commit fixes the oversight.
mtreinish/stestr,masayukig/stestr,mtreinish/stestr,masayukig/stestr