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

Signed-off-by: Leonardo Garcia <lagarcia@br.ibm.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <7f13e40e05ddb411697b0777b0e37757f76905e9.1638982486.git.lagarcia@br.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

diff --git a/docs/specs/ppc-spapr-hcalls.txt b/docs/specs/ppc-spapr-hcalls.rst
similarity index 100%
rename from docs/specs/ppc-spapr-hcalls.txt
rename to docs/specs/ppc-spapr-hcalls.rst