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

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>