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The yum command has been replaced by the dnf command in recent releases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/plugins.rst b/doc/source/plugins.rst
index dd75b5a..fe567e2 100644
--- a/doc/source/plugins.rst
+++ b/doc/source/plugins.rst
@@ -232,7 +232,7 @@
 plugin as files that contain new-line separated lists of packages
 required by the plugin
 
-Supported packaging systems include apt and yum across multiple
+Supported packaging systems include apt and dnf across multiple
 distributions.  To enable a plugin to hook into this and install
 package dependencies, packages may be listed at the following
 locations in the top-level of the plugin repository: