Documentation: admin-guide: tainted-kernels.rst: Add missing article and comma

- Add missing article "the"
- s/above example/example above/
- Add missing comma after introductory clause to improve readability

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240205132409.1957-1-thorsten.blum@toblux.com