Fix the example command for running a unit test.
(Colon versus period.)
Fix box formatting for tox commands. (Need extra
newline after double colon.)
Fix a typo in the same section.
Fix broken ostestr doc link.
Remove testr and flake8 links that don't go anywhere.
Change-Id: I243e4f0639ef4a87e8950e0428b7556919094b0e
2828bfcfb0