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>

scripts/sphinx-pre-install

@@ -371,7 +371,7 @@ sub give_redhat_hints()
 	#
 	# Checks valid for RHEL/CentOS version 7.x.
 	#
-	if (! $system_release =~ /Fedora/) {
+	if (!($system_release =~ /Fedora/)) {
 		$map{"virtualenv"} = "python-virtualenv";
 	}