Minor: typo fix

Signed-off-by: Sam Kleinman <samk@10gen.com>
Update install-mongodb-on-red-hat-centos-or-fedora-linux.txt

I couldn't receive the xml file from 10gen repo. I added the final "/" in "baseurl=http://downloads-distro.mongodb.org/repo/redhat/os/x86_64/" and it worked

Signed-off-by: Sam Kleinman <samk@10gen.com>