linux-phytec-ti

docs: ubifs-authentication.md: convert to ReST

The documentation standard is ReST and not markdown.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>