qga: Fix guest-network-get-route return value documentation

Tagged sections are only recognized at the beginning of a paragraph.
guest-network-get-route's Returns: isn't, and therefore gets rendered
as ordinary text within its paragraph:

    Retrieve information about route of network. Returns: List of route
    info of guest.

Since there is no (recognized) Returns: section, the doc generator
adds

    Return:
       [GuestNetworkRoute]

Note: only since recent commit 636c96cd77 (qapi: Fix undocumented
return values by generating something).

Insert the required blank line so that Returns: is recognized.
Result:

    Retrieve information about route of network.

    Return:
       [GuestNetworkRoute] -- List of route info of guest.

Fixes: commit 8e326d36dd (qga/linux: Add new api 'guest-network-get-route')
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250717115246.3830007-2-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

qga/qapi-schema.json

@@ -1966,6 +1966,7 @@
 # @guest-network-get-route:
 #
 # Retrieve information about route of network.
+#
 # Returns: List of route info of guest.
 #
 # Since: 9.1