transparent-gateway.rst: Fix formatting for numbered list

Without the extra empty line the formatting for the numbered would not
kick in.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

docs/blueprints/transparent-gateway.rst

@@ -12,6 +12,7 @@ Overview
 ********
 
 The Transparent Gateway Blueprint provides support for building a PoC gateway that can be the communication hub in a smart home. Some of the features it demonstrates are:
+
 #. Ability to participate in an OpenThread network
 #. Automatic IPv6-to-IPv4 translation between devices and rest of the world
 #. Wifi AP functionality