doc: Use dnf instead of yum

The yum command has been replaced by the dnf command in recent releases
of Fedora-based distributions. Use the native command instead of
the alias kept for compatibility.

Change-Id: I0a1dfdaca91164eff2c25795f66976ec70356574
diff --git a/doc/source/guides/single-machine.rst b/doc/source/guides/single-machine.rst
index a4385b5..263fbb9 100644
--- a/doc/source/guides/single-machine.rst
+++ b/doc/source/guides/single-machine.rst
@@ -62,7 +62,7 @@
 
 .. code-block:: console
 
-    $ apt-get install sudo -y || yum install -y sudo
+    $ apt-get install sudo -y || dnf install -y sudo
     $ echo "stack ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/stack
 
 .. note:: On some systems you may need to use ``sudo visudo``.
@@ -81,7 +81,7 @@
 
 .. code-block:: console
 
-    $ sudo apt-get install git -y || sudo yum install -y git
+    $ sudo apt-get install git -y || sudo dnf install -y git
     $ git clone https://opendev.org/openstack/devstack
     $ cd devstack