scripts/sphinx-pre-install: fix script for RHEL/CentOS

scripts/sphinx-pre-install: fix script for RHEL/CentOS

BugLink: https://bugs.launchpad.net/bugs/1840520
[ Upstream commit b308467c916aa7acc5069802ab76a9f657434701 ]

There's a missing parenthesis at the script, with causes it to
fail to detect non-Fedora releases (e. g. RHEL/CentOS).

Tested with Centos 7.6.1810.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>