Documentation: ubifs: Fix compression idiom

Clearly the author meant 'on the fly'.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20221005030904.65604-1-joel@jms.id.au
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: filesystems: convert ubifs.txt to ReST

- Add a SPDX header;
- Add a document title;
- Adjust section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add lists markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9043dc2965cafc64e6a521e2317c00ecc8303bf6.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>