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>

README

@@ -11,7 +11,7 @@ In order to build the documentation, use ``make htmldocs`` or
     https://www.kernel.org/doc/html/latest/
 
 There are various text files in the Documentation/ subdirectory,
-several of them using the ReStructured Text markup notation.
+several of them using the reStructuredText markup notation.
 
 Please read the Documentation/process/changes.rst file, as it contains the
 requirements for building and running the kernel, and information about