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The yum command has been replaced by the dnf command in recent releases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/multinode-lab.rst b/doc/source/guides/multinode-lab.rst
index 658422b..4b50b2c 100644
--- a/doc/source/guides/multinode-lab.rst
+++ b/doc/source/guides/multinode-lab.rst
@@ -24,7 +24,7 @@
 
 ::
 
-    apt-get install -y git sudo || yum install -y git sudo
+    apt-get install -y git sudo || dnf install -y git sudo
 
 Network Configuration
 ---------------------