docs: fix make rpm sphinx error

On `make rpm`, docs build fails with:

    docs/source/optional_plugins/varianter_pict.rst:33: WARNING: Could
    not lex literal_block as "ini".

This patch changes the way to define a block of code for a
proven-to-be-good one.
Signed-off-by: NAmador Pahim <apahim@redhat.com>

docs/source/optional_plugins/varianter_pict.rst

@@ -28,15 +28,13 @@ is not as convenient as having it on your ``$PATH``.
 Using the PICT Varianter Plugin
 -------------------------------
 
-The following listing is a sample (simple) PICT file.
+The following listing is a sample (simple) PICT file::
 
-.. code-block:: ini
-
-     1  arch: intel, amd
-     2  block_driver: scsi, ide, virtio
-     3  net_driver: rtl8139, e1000, virtio
-     4  guest: windows, linux
-     5  host: rhel6, rhel7, rhel8
+    arch: intel, amd
+    block_driver: scsi, ide, virtio
+    net_driver: rtl8139, e1000, virtio
+    guest: windows, linux
+    host: rhel6, rhel7, rhel8
 
 To list the variants generated with the default combination order (2,
 that is, do a pairwise idenpendent combinatorial testing)::