Documentation: change linux-4.x references to 5.x

As linux-5.0.x is coming up soon, the documentation should match,
in particular the README.rst file, so change all 4.x references
accordingly. There was a mix of lowercase and uppercase X here,
which I changed to using lowercase consistently.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>