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The yum command has been replaced by the dnf command in recent releases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-vm.rst b/doc/source/guides/single-vm.rst
index 7dac18b..4272a4b 100644
--- a/doc/source/guides/single-vm.rst
+++ b/doc/source/guides/single-vm.rst
@@ -56,8 +56,8 @@
     write_files:
       - content: |
             #!/bin/sh
-            DEBIAN_FRONTEND=noninteractive sudo apt-get -qqy update || sudo yum update -qy
-            DEBIAN_FRONTEND=noninteractive sudo apt-get install -qqy git || sudo yum install -qy git
+            DEBIAN_FRONTEND=noninteractive sudo apt-get -qqy update || sudo dnf update -qy
+            DEBIAN_FRONTEND=noninteractive sudo apt-get install -qqy git || sudo dnf install -qy git
             sudo chown stack:stack /home/stack
             cd /home/stack
             git clone https://opendev.org/openstack/devstack