docs: filesystems: link ubifs-authentication.rst without .rst extension

Specifying the .rst extension doesn't cause any problems AFAICT, but
it's uncommon.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20201108132415.1789142-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>