Fix typo with log_config in zuul.conf-sample

Change-Id: Ieda682233843b4500e8d59d91ac24b36dfd172dd
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-on: https://review.openstack.org/14433
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins

etc/zuul.conf-sample

@@ -10,7 +10,7 @@ sshkey=/home/jenkins/.ssh/id_rsa
 
 [zuul]
 layout_config=/etc/zuul/layout.yaml
-log_config=/etc/zuul/logging.yaml
+log_config=/etc/zuul/logging.conf
 pidfile=/var/run/zuul/zuul.pid
 state_dir=/var/lib/zuul
 git_dir=/var/lib/zuul/git