Add sudo to yum example

Add sudo to yum example so following along with copy-paste works

Change-Id: I5e64b3d751b55989a353bfe2bb691ea6e51690e3
diff --git a/doc/source/guides/single-machine.rst b/doc/source/guides/single-machine.rst
index 70287a9..236ece9 100644
--- a/doc/source/guides/single-machine.rst
+++ b/doc/source/guides/single-machine.rst
@@ -67,7 +67,7 @@
 
 ::
 
-    sudo apt-get install git -y || yum install -y git
+    sudo apt-get install git -y || sudo yum install -y git
     git clone https://git.openstack.org/openstack-dev/devstack
     cd devstack