Update URL to oo-diagnostics in docs.

Update the URL to oo-diagnostics in the documentation.

Github issue: https://github.com/openshift/origin-server/issues/5662

- oo-diagnostics has been moved from util to commin/bin. See: https://github.com/openshift/origin-server/commit/97b13c1787aab28115179ecbfd3c0cac21d500be
- Github now uses githubusercontent.com to serve some content.