docs: filesystems: convert hfsplus.txt to ReST

Just trivial changes:

- Add a SPDX header;
- Add it to filesystems/index.rst.

While here, adjust document title, just to make it use the same
style of the other docs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4298409da951fbee000201a6c8d9c85e961b2b79.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>