docs: update 'Generic installation'

Let's update the command to install the packages dependencies to
the latest fedora. In this commit:

- Switch from 'yum' to 'dnf'.
- Add python2: latest Fedora does not ship python2 package in minimal
installation.
- Add libffi-devel and openssl-devel: not shipped in latest Fedora
minimal installation.
Signed-off-by: NAmador Pahim <apahim@redhat.com>

docs/source/GetStartedGuide.rst

@@ -134,7 +134,7 @@ First make sure you have a basic set of packages installed. The
 following applies to Fedora based distributions, please adapt to
 your platform::
 
-    sudo yum install -y git gcc python-devel python-pip libvirt-devel libyaml-devel redhat-rpm-config xz-devel
+    sudo dnf install -y python2 git gcc python-devel python-pip libvirt-devel libffi-devel openssl-devel libyaml-devel redhat-rpm-config xz-devel
 
 Then to install Avocado from the git repository run::