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The yum command has been replaced by the dnf command in recent releases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: I460202c809b442b8b0a00008162ca664db638f59

doc/source/install/install-rdo.rst

@@ -329,7 +329,7 @@ Install and configure components
 
    .. code-block:: console
 
-      # yum install openstack-heat-api openstack-heat-api-cfn \
+      # dnf install openstack-heat-api openstack-heat-api-cfn \
         openstack-heat-engine
 
 2. Edit the ``/etc/heat/heat.conf`` file and complete the following