Fix the following warning

Documentation/bpf/ringbuf.rst: WARNING: document isn't
included in any toctree

By adding it to the index.
Signed-off-by: NDaniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-10-dwlsalmeida@gmail.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>