docs: misc-devices/apds990x.txt: convert to ReST format

- Adjust title markup;
- Adjust identation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/444743782e5c8b8085d8016c8be3baabe4d23b53.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>