docs: Rename ppc-spapr-hotplug.txt to ppc-spapr-hotplug.rst.

Signed-off-by: Leonardo Garcia <lagarcia@br.ibm.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <1f5860217273f272fddadc68b5d205b4090f6b04.1641995058.git.lagarcia@br.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>