Documentation: net: net_failover: Separate cloud-ifupdown-helper and reattach-vf.sh code blocks marker

cloud-ifupdown-helper patch and reattach-vf.sh script are rendered in
htmldocs output as normal paragraphs instead of literal code blocks
due to missing separator from respective code block marker. Add it.

Signed-off-by: Bagas Sanjaya <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/net_failover.rst

···
 received only on the 'failover' device.
 
 Below is the patch snippet used with 'cloud-ifupdown-helper' script found on
-Debian cloud images:
+Debian cloud images::
 
-::
   @@ -27,6 +27,8 @@ do_setup() {
        local working="$cfgdir/.$INTERFACE"
        local final="$cfgdir/$INTERFACE"
···
 
 The following script is executed on the destination hypervisor once migration
 completes, and it reattaches the VF to the VM and brings down the virtio-net
-interface.
+interface::
 
-::
   # reattach-vf.sh
   #!/bin/bash