README: Fix spelling

- ReStructured Text should be exactly reStructuredText
- "reStructuredText" is ONE word, not two! according to https://docutils.sourceforge.io/rst.html



Signed-off-by: Maki Hatano <Maki.Y.Hatano@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20240313100136.20424-1-Maki.Y.Hatano@gmail.com>