[stable-only] reno: fix indentation

This missing empty line causes errors time to time. The simplest is to
fix it on every branch so that this won't come up again and again in
the future.

The error is usually something like this:

<reno.sphinxext <some_branch>>:32:Unexpected indentation.

And can be identified with adding extra verbosity to releasenotes job,
via -vvv argument in tox.ini to the sphinx-build command.

Change-Id: I17da28b3ed181531e32a3c2ce3ef445af1d5c978
(cherry picked from commit 254a03a0f2)
(cherry picked from commit 559da73672)
(cherry picked from commit 3de04a4a8c)

releasenotes/notes/bp-port-mirroring-sriov-vf-879bc2aa53c2c8d4.yaml

@@ -5,6 +5,7 @@ prelude: >
 features:
   - |
     [`blueprint port-mirroring-sriov-vf <https://blueprints.launchpad.net/neutron/+spec/port-mirroring-sriov-vf>`_]
+
     * Adds a new TaaS Agent driver (i.e. SRIOV) for Intel i40e driver backend.
     * Neutron TaaS Agent is refactored to decouple its tight binding with ovs
       driver. This prepares the taas agent code-base for addition of a new