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The yum command has been replaced by the dnf command in recent releases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/networking.rst b/doc/source/networking.rst
index 05b4f34..10e1c3f 100644
--- a/doc/source/networking.rst
+++ b/doc/source/networking.rst
@@ -213,7 +213,7 @@
 
 .. code-block:: bash
 
-    sudo apt-get install sshuttle || yum install sshuttle
+    sudo apt-get install sshuttle || dnf install sshuttle
 
 Finally, start ``sshuttle`` on your localhost using the floating IP address
 range. For example, assuming you are using the default value for