Adapt template to towncrier 23.11.0

It generated error:
Warning, treated as error:
/buildbot/buildbot-job/build/master/docs/relnotes/index.rst:25:Bullet list ends without a blank line; unexpected unindent.
make[1]: *** [Makefile:58: html] Error 2

See https://github.com/twisted/towncrier/releases/tag/23.10.0
Two newlines are no longer always added between the current release notes and the previous content. The newlines are now defined only inside the template.

Important! If you're using a custom template and want to keep the same whitespace between releases, you may have to modify your template. (buildbot#552)

twisted/towncrier#552

master/docs/relnotes.rst.jinja

@@ -15,6 +15,7 @@
 {% for text, values in sections[section][category]|dictsort(by='value') %}
 - {{ text }}
 {% endfor %}
+
 {% else %}
 - {{ sections[section][category]['']|sort|join(', ') }}