docs: Neutron - add required newline for code formatting

Otherwise it doesn't render it as the correct HTML tag.

Change-Id: Ibc5c846824c7248c64473512e9dcfe5dd4205924
diff --git a/doc/source/guides/neutron.rst b/doc/source/guides/neutron.rst
index 1e20d7f..c6af953 100644
--- a/doc/source/guides/neutron.rst
+++ b/doc/source/guides/neutron.rst
@@ -448,6 +448,7 @@
 the `localrc` part of `local.conf` on each machine.
 
 ::
+
     Q_ML2_PLUGIN_PATH_MTU=1500